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Thd  U.C.  System 

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UNIVERSITY       OF       CALIFORNIA 


Edited  by 
KENNETH  F.  BAKER 


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The  U.  C.  System 
for  Producing  Healthy 

Container-Grown  Plants 


through  the  Use  of  Clean  Soil, 
Clean  Stock,  and  Sanitation 


Edited  by  KENNETH   F.   BAKER 

PHILIP  A.  CHANDLER,  RICHARD  D,  DURBIN,  JOHN  FERGUSON, 
J.  W.  HUFFMAN,  O.  A.  MATKIN,  DONALD  E.  MUNNECKE,  CHESTER 
N.  ROISTACHER,  WARREN  R.  SCHOONOVER,  and  R.  H.  SCIARONI 


UNIVERSITY  OF  CALIFORNIA   DIVISION  OF  AGRICULTURAL  SCIENCES 
AGRICULTURAL  EXPERIMENT  STATION— EXTENSION  SERVICE 


THIS  MANUAL  is  one  of  a  series  published  by  the  University  of  California  College  of 
Agriculture  and  sold  for  a  charge  which  is  based  upon  returning  only  a  portion  of  the 
production  cost.  I>\  this  means  it  is  possible  to  make  available  publications  which,  due  to 
relatively  high  cost  of  production,  or  limited  audience,  would  otherwise  be  beyond  the 
scope  of  the  College  publishing  program. 


CONTENTS 

Page 

1.  The  U.  C.  System:  a  General  Summary,  by  Kenneth  F.  Baker 3 

The  problems;  the  answers;  the  future. 

2.  Today's  Nursery  Problems,  by  Kenneth  F.  Baker 28 

The  industry;  plant  diseases;  adopting  the  U.  C.  system. 

3.  Damping-off  and  Related  Diseases,  by  Kenneth  F.  Baker 34 

Dynamics  and  prevention  of  nursery  diseases. 

4.  The  Salinity  Problem  in  Nurseries,  by  Warren  R.  Schoonover  and  R.  H.  Sciaroni 52 

Nature;  plant  injury;  cause;  prevention. 

5.  The  U.  C.-Type  Soil  Mixes,  by  O.  A.  Matkin  and  Philip  A.  Chandler 68 

Preparation  and  uses  of  soil  mixes  and  fertilizers. 

6.  Components  and  Development  of  Mixes,  by  O.  A.  Matkin,  Philip  A.  Chandler,  and 
Kenneth  F.  Baker 86 

Selecting  physical  and  chemical  components;  history. 

7.  Nitrogen  in  Nursery  Soils,  by  O.  A.  Matkin  and  Philip  A.  Chandler 108 

Types  and  utilization  of  nitrogen  sources. 

8.  Heat  Treatment  of  Soil,  by  Kenneth  F.  Baker  and  Chester  N.  Roistacher 123 

Benefits;  practical  procedures;  cost. 

9.  Principles  of  Heat  Treatment  of  Soil,  by  Kenneth  F.  Baker  and  Chester  N.  Roistacher.  ...    138 

How  soil  is  heated;  factors  in  equipment  design. 

10.  Equipment  for  Heat  Treatment  of  Soil,  by  Kenneth  F.  Baker  and  Chester  N.  Roistacher.  .    162 

Selecting  equipment;  steam  generators;  fuel. 

1 1.  Chemical  Treatment  of  Nursery  Soils,  by  Donald  E.  Munnecke 197 

Materials  and  methods;  soil  drenches. 

12.  Treatment  of  Nursery  Containers,  by  Kenneth  F.  Baker,  Chester  N.  Roistacher,  and 

Philip  A.  Chandler 210 

Heat;  chemicals;  self-disinfesting  containers. 

13.  Development  and  Maintenance  of  Healthy  Planting  Stock,  by  Kenneth  F.  Baker  and 

Philip  A.  Chandler 217 

Significance;  obtaining  and  maintaining  stock. 

14.  Beneficial  Soil  Microorganisms,  by  John  Ferguson 237 

Types;  activities;  controlled  colonization. 

15.  Importance  of  Variation  and  Quantity  of  Pathogens,  by  Richard  D.  Durbin 255 

Variability;  inoculum  potential;  longevity  in  soil. 

16.  Grower  Experience  with  the  U.  C.  System,  by  R.  H.  Sciaroni  and  J.  W.  Huffman 263 

Report  on  12  examples  of  7  types  of  nurseries. 

17.  Mechanization  and  the  U.  C.  System,  by  J.  W.  Huffman  and  R.  H.  Sciaroni 271 

Steps  in  production;  flow  diagrams. 

Appendix    285 

References;  glossary;  weights  and  measures;  sources  of  equipment, 
materials,  fungicides,  and  chemicals. 

Index     307 


THE  U.  C.  SYSTEM 


THE  AUTHORS 

Kenneth  F.  Baker,  Professor  of  Plant  Pathology,  and 
Plant  Pathologist  in  the  Experiment  Station,  Los 
Angeles. 

Philip  A.  Chandler,  Principal  Laboratory  Technician, 
Department  of  Plant  Pathology,  Los  Angeles. 

Richard  D.  Durbin,  Senior  Laboratory  Technician, 
Department   of    Plant    Pathology,    Los   Angeles. 

John  Ferguson,  former  Research  Assistant,  Depart- 
ment of  Plant  Pathology,  Los  Angeles;  now  at  the 
Soil    and    Plant    Laboratory,    Orange,    California. 

John  W.  Huffman,  Assistant  Agriculturist,  Agricul- 
tural   Extension   Ssrvice,   Los  Angeles. 

O.  A.  Matkin,  former  graduate  student  in   Horticul- 


tural Science,  University  of  California,  Los  Ange- 
les; now  at  the  Soil  and  Plant  Laboratory, 
Orange,   California. 

Donald  E.  Munnecke,  Assistant  Professor  of  Plant 
Pathology  and  Assistant  Plant  Pathologist  in  the 
Experiment   Station,   Los  Angeles. 

Chester  N.  Roistacher,  Principal  Laboratory  Techni- 
cian, Department  of  Plant  Pathology,  Citrus  Ex- 
periment Station,   Riverside,   California. 

Warren  R.  Schoonover,  Agriculturist,  Agricultural 
Extension    Service,    Berkeley. 

R.  H.  Sciaroni,  Associate  Agriculturist,  Agricultural 
Extension   Service,   Half  Moon   Bay. 


Aids  in  Adopting 
the  U.  C.  System 


here  are  several  logical  steps  in  con- 
sidering the  adoption  of  the  system, 
whether  in  converting  an  old  nursery  or 
establishing  a  new  one. 

Decide  whether  the  U.  C.  system  for 
producing  healthy  plants  is  to  be  adopted 
in  your  nursery.  The  experiences  of 
twelve  growers  who  have  done  so  are 
described  in  Section  16.  The  advantages 
of  such  adoption  are  summarized  on  p. 
30  through  33,  49,  51,  and  270.  If  the 
system  is  to  be  adopted,  the  cultural  prac- 
tices, present  and  contemplated,  should 
be  surveyed  on  the  following  bases: 

1.  Compare  the  U.  C.-type  soil  mixes 
and  nutrients  with  other  possible  soils  on 
the  bases  indicated  in  figure  65  and  in 
the  related  discussion  in  Section  6.  A 
summary  may  be  found  on  p.  90,  93, 
and  94. 

2.  The  comparative  ease  of  avoiding 
salinity  injury  by  methods  outlined  in 
Section  4  should  be  studied  for  each  of 
the  cultural  systems  considered. 

3.  Compare  results  with  untreated  soil 
and  that  treated  with  heat  or  chemicals. 
The  experiences  of  the  twelve  growers 
described  in  Section  16,  and  the  benefits 
reported  on  p.  49  and  51  show  the  neces- 
sity of  soil  treatment. 

4.  Compare  treatment  of  soil  by  chem- 
icals and  by  steam  to  determine  which 
fits  best  into  your  operations.  See  table 
13  and  the  accompanying  discussion  in 
Section  8. 

5.  If  steam  is  selected  for  soil  treat- 
ment: 

a.  The  boiler  size  required  for  a  given 
amount  of  soil  may  be  estimated  from 
table  14  and  the  discussion  in  Section  9. 

b.  The  type  of  equipment  best  suited 


for  steaming  in  your  nursery  may  be  se- 
lected by  consulting  table  15  and  the  data 
in  Section  10. 

c.  Possible  ways  to  integrate  such 
equipment  into  the  mechanization  pro- 
gram are  shown  in  figure  126  and  the 
text  of  Section  17. 

6.  If  chemicals  are  to  be  used  for  soil 
treatment,  the  one  best  suited  to  your 
needs  may  be  determined  through  table 
17  and  the  discussion  in  Section  11.  Ways 
to  integrate  such  a  treatment  into  a  me- 
chanization program  are  shown  in  figure 
126  and  the  text  of  Section  17. 

7.  Possible  methods  for  treating  con- 
tainers are  discussed  in  Section  12. 
Whenever  possible,  however,  the  soil 
should  be  treated  in  the  containers. 

8.  Pathogen-free  stock  or  seed  is  neces- 
sary. Methods  for  obtaining  and  main- 
taining it  are  outlined  in  figure  115  and 
in  the  text  of  Section  13. 

9.  Sanitary  precautions  necessary  to 
prevent  contamination  of  clean  soil  and 
stock  are  outlined  in  "A  Nursery  Sanita- 
tion Code"  in  Section  1.  The  entire  oper- 
ation should  be  reviewed,  step  by  step, 
from  the  viewpoint  of  eliminating  all 
possible  sources  of  contamination. 

10.  Some  possibilities  of  mechaniza- 
tion are  presented  in  the  flow  diagram 
(fig.  126)  and  in  the  accompanying  text 
in  Section  17.  Such  a  chart  might  well  be 
prepared  for  the  contemplated  plan.  You 
should  also  visit  mechanized  nurseries 
and  perhaps  consult  an  engineer  who 
specializes  in  materials  handling. 

11.  The  technical  assistance  and  ad- 
vice of  a  well-trained  person  will  reduce 
errors  and  ease  the  transition  while  the 
system  is  being  adopted.  In  some  coun- 


[i] 


ties  your  farm  advisor  will  be  able  to  stand  why  the  various  practices  are  per- 

help   during  this   "shake-down"   period.  formed.     This     manual     should     prove 

12.  The  employees  should  be  trained  helpful  in  such  training. 
in  the  system  used  so  that  they  under- 


No  endorsement  of  products  or  equipment  referred  to  by  trade  names  in  this  manual  is  intended,  nor  is 
criticism  implied  of  similar  products  which  are  not  mentioned.  No  responsibility  is  assumed  for  commercial 
soil    mixes  or   fertilizers   sold    as   formulations   presented    in   this   publication. 


SECTION 


The  U.C.  System: 
A  General  Summary 


The  problems 


The  answers 


The  future 


Kenneth  F.  Baker 


.he  most  urgent  need  of  the  Califor- 
nia nursery  industry,  within  the  limits  of 
its  present  market,  is  for  lowered  cost  of 
production.  This  is  best  achieved  by  re- 
ducing plant  losses  and  by  lowering  labor 
cost  through  mechanization.  These  in 
turn  require  modification  of  many  exist- 
ing practices.  Production  must  be  de- 
pendable, uniform,  and  largely  free  from 
unpredictable  failures  due  to  diseases, 
salinity,  insects,  or  weather. 

The  U.  C.  system  of  soil  mixes,  soil 
and  plant  treatments,  and  handling  op- 
erations has  been  developed  since  1941 
by  the  Department  of  Plant  Pathology, 
University  of  California,  Los  Angeles, 
to  practically  eliminate  the  principal 
cause  of  such  failure — diseases  caused 
by  those  organisms  and  factors  which 
involve  the  soil.  It  was  evolved  during 
a  time  when  mechanization  was  empha- 
sized and,  therefore,  provides  the  neces- 
sary adaptability  and  dependability  for 
the  success  of  such  a  program.  (Sec.  2.)1 

1  These  numbers  in  parentheses  give  the  sec- 
tion where  further  information  on  the  topic 
may  be  found. 


Growers  have  generally  found  that 
they  can  produce  better  plants  faster, 
easier,  and  more  dependably  by  the  U.  C. 
system  than  by  previous  methods.  Conse- 
quently many  growers  in  California,  as 
well  as  in  other  areas,  have  adopted  the 
system  or  parts  of  it,  and  have  contrib- 
uted in  turn  to  its  development.  The 
large,  year-round  demand  for  nursery 
stock  in  California  has  given  added  im- 
petus to  the  program.  The  real  key  to  the 
development  and  adoption  of  the  system, 
however,  has  been  the  effective  disease 
and  salinity  control  it  has  provided. 
(Sec.  16.) 

The  magnitude  of  the  over-all  problem 
is  indicated  by  the  quantity  of  soil  used 
by  the  California  nursery  industry  each 
year.  It  is  estimated  that  this  is  about 
350,000  cubic  yards,  or  the  top  foot  of 
soil  from  217  acres  of  land.  (Sec.  2.) 

Advances  in  disease  control 

There  has  been  close  parallel  develop- 
ment of  disease-control  practices  in 
plants  and  animals.  Both  have  progressed 
from  superstitious  practices,  through  the 


[3] 


use  of  antiseptics  (on  animals)  and 
sprays  (on  plants),  to  aseptic  proce- 
dures, to  use  of  antibiotics  (on  animals 
and  plants)  and  retardant  or  antagon- 
istic organisms  (in  soil).  With  plants 
this  has  emphasized  pathogen-free  soil 
and  propagative  material,  and  cultural 
techniques  to  keep  them  that  way.  This, 
the  central  core  of  the  U.  C.  system,  is 
expressed  in  the  motto,  "Don't  fight  'em, 
eliminate  'em."  (Sees.  3,  13,  and  14.) 

Successful  prevention  of  a  disease  usu- 
ally involves  the  application  of  more 
than  one  treatment.  For  this  reason,  sev- 
eral concurrent  procedures  are  often  sug- 
gested for  a  given  trouble  in  this  manual. 

Objective  of  this  manual 

The  objective  of  this  publication  is  to 
assemble  and  synthesize  information 
from  many  sources  into  a  unified  plan 
of  action  for  nurseries.  Many  results  of 
our  research  are  reported  for  the  first 
time.  In  addition,  selected  and  tested  in- 
formation from  many  other  sources  has 
been  included  to  give  a  reasonably  com- 
plete picture  up  to  January,  1957.  This 
manual  is  based  on,  and  perhaps  will 
prove  most  useful  to  California  nurseries, 
but  it  should  be  helpful  to  growers  in 
other  areas  who  have  shown  interest  in 
the  U.  C.  system. 

While  specific  recommendations  of 
present  general  application  are  made  in 


this  manual,  it  is  not  a  "cookbook"  of 
exact  instructions.  Nurseries  vary  so 
widely  in  type,  size,  and  location,  and 
in  the  number,  kind,  and  age  of  crops 
grown,  that  detailed  recommendations 
are  often  of  limited  application.  Further- 
more it  is  impossible  to  anticipate  the 
course  of  future  developments  in  nursery 
practice.  If  growers  understand  the  sci- 
entific bases  underlying  their  practices, 
they  can  adapt  the  system  to  their  own 
conditions  and  to  future  improvements 
in  methods  and  equipment.  The  dual 
purpose  of  the  manual,  then,  is  to 
provide  a  plan  for  present  action,  with 
background  information  that  will  help 
the  grower  shift  with  the  improvements 
that  will  surely  come. 

One  of  the  principal  uses  of  this 
manual  probably  will  be  as  a  reference 
work  to  look  up  some  specific  fact,  and 
the  format  has  been  arranged  with  this 
in  mind.  A  fairly  complete  index,  num- 
erous headings,  frequent  cross-refer- 
ences, tabulated  data,  and  a  general 
summary  are  provided.  The  same  infor- 
mation may  appear  more  than  once,  in 
order  to  reduce  the  danger  that  the  refer- 
ence user  may  acquire  partial  facts  or 
facts  out  of  context.  It  has  also  some- 
times been  necessary  to  repeat  points  in 
order  to  develop  a  line  of  reasoning.  It 
is  hoped  that  the  annoyance  to  the  gen- 
eral reader  from  such  repetition  will  be 
outweighed  by  its  general  utility. 


THE  PROBLEMS 

Diseases 

Diseases  are  a  luxury 

Nurserymen  are  coming  to  realize 
that  diseases  are  neither  a  necessary  evil 
nor  a  trivial  factor  in  the  gamble  in- 
volved in  production  of  a  crop.  Diseases 

are,   in   fact,   important  in   determining     The  pathogens,  and 
what  the  grower  does,  how  and  when  he      n$Wtn«iy  jsproad 
docs  it,  and  why.  The  viewpoint  is  gain- 
ing acceptance  that  diseases  are  a  luxury 
modern  nurseries  can  ill  afford,  because 
I  lies    caii-c   unnecessary   losses.   Because 


of  the  trend  toward  crop  specialization, 
it  is  also  well  to  remember  that  the  fewer 
the  crops  grown,  the  less  can  one  afford 
erratic  disease  loss  in  them.  (Sees.  2 
and  3.) 


Damping-off  (figs.  1,  12  through  18, 
30,  33,  and  34)  and  related  diseases 
(such  as  seed  decay,  top  rot,  cutting  and 
stem  rot,  and  root  rot  of  mature  plants) 


I  4 


of  nursery  crops  are  most  frequently 
caused  by  Rhizoctonia  solani,  but  also  by 
water  molds  (Pythium  and  Phytoph- 
thora  spp.).  Less  important  are  the  cot- 
tony-rot fungi  (Sclerotinia  sclerotiorum 
and  S.  minor)  and  the  gray  mold  (Bo- 
trytis  cinerea) ,  as  well  as  host-special- 
ized parasites,  such  as  the  aster-wilt  Fu- 
sarium.   (Sec.  3.) 

The  losses  caused  by  a  disease  are  de- 
termined by  the  interaction  of  several 
factors:  (1)  the  susceptibility  of  the 
host,  as  well  as  its  carbohydrate-nitrogen 
status,  and  the  vitality  of  the  seed 
planted;  (2)  the  abundance  (inoculum 
potential)2  and  virulence  of  the  patho- 
gen in  the  soil;  (3)  the  favorableness  of 
the  environment  (for  example,  the  levels 
of  salinity,  moisture,  and  temperature  in 
the  soil,  the  light  available  to  the  plant, 
the  depth  of  planting).  (Sec.  3.) 

The  destructive  Rhizoctonia  and  water 
molds  produce  no  important  air-borne 
spore  stage,  and  their  spread  is,  there- 
fore, largely  dependent  on  the  scattering 

2  Many  of  the  technical  terms  used  in  this 
manual  are  explained  in  the  Glossary  (p.  298). 


CAUTION: 

Many 

of 

the  < 

:hemicals 

mentioned 

in  this 

manual 

are 

poi- 

sonous   and 

may 

be 

harmful. 

The 

user  should 

carefu 

lly 

Follow  the 

pre- 

cautions  on 

the   1 

abe 

Is  of 

the 

con- 

tainers. 

of  soil  or  plant  fragments  in  which  they 
are  present.  Spores  of  the  water  molds 
may  be  spread  in  water  stored  in  a  tank 
or  reservoir,  but  not  in  that  from  city 
mains  or  wells.  Spread  of  any  of  these 
fungi  may  occur  when: 

1.  Soil  is  spattered  by  drops  or  a  jet  of 

water,  as  from  irrigation  or  rain; 

2.  The   pathogen   is   spread   by   dipping 

cuttings  in  water  or  in  hormone 
solutions; 

3.  Soil  gets  in  the  end  of  the  hose  when 

it  is  dropped  on  the  ground,  and 
is  expelled  into  the  bench  with  the 
next  watering; 

4.  Soil    is    carried    over    on    flats,    pots, 

benches,  or  other  containers  be- 
tween plantings; 


Fig.  1.  Rhizoctonia  damping-off  of  pepper  seedlings.  Flat  at  left  showing  damping-off  pro- 
gressing inwardly  from  an  infested  container.  Next  to  the  flat  the  seedlings  were  infected  first, 
and  rotted  before  emergence.  Farther  from  the  edge,  the  seedlings  were  successively  bigger 
before  postemergence  damping-off  occurred.  At  right  is  shown  the  wire-stem  phase  of  damping- 
off  (see  arrow)  of  large  seedlings,  causing  the  plants  to  fall  over. 


Fig.  2.  Which  is  the  most  important  leg  of  a 
three-legged  stool? 

5.  Soil   is   carried   over   on   tools,   cloth 

covers  for  flats,  and  on  workmen's 
hands; 

6.  The  grower  walks  over  treated  soil  or 

flats; 

7.  Flats  are  placed  on  infested  ground; 

8.  Infected   seed,   cuttings,   or   seedlings 

are  planted. 

Therefore,  emphasis  in  control  is  placed 
on  clean  soil,  clean  stock,  and  sanitary 
procedures  to  keep  them  that  way  (fig. 
2 ) .  Once  the  pathogen  has  penetrated 
into  a  plant  it  is  not  economically  pos- 
sible to  eradicate  it  except,  with  valuable 
planting  stocks,  by  heat  treatment.  (Sec. 
13.)  Chemical  treatments  generally  are 
ineffective.  For  these  reasons  prevention 
is  emphasized  in  plant  disease  control, 
rather  than  cure,  as  in  medical  pro- 
cedures. (Sec.  3.) 

These  fungi  are  not 
restricted  to  juvenile  plants 

Fungi  which  cause  damping-off  and 
related  seedling  diseases  may  kill  a  tree 
or  shrub  some  years  later  when  the  loss 
is  greater,  or  may  infest  for  all  time  a 
clean  piece  of  ground.  This  fact  imposes 
on  nurserymen  the  obligation  to  produce 
plants  free  of  disease  organisms,  rather 
than  merely  free  of  symptoms.  Cultural 
suppression   of   these  soil   fungi   in   the 


nursery  is  only  likely  to  postpone  dis- 
ease until  later,  costlier  losses  occur. 
(Sec.  3.) 

Two  strains  of  one 
species  may  differ  greatly 

There  is  a  widespread  misconception 
that,  because  Rhizoctonia  solani  (or some 
other  microorganism)  is  naturally  pres- 
ent in  a  given  soil,  it  is  unimportant 
whether  more  of  it  is  introduced  with 
planting  stock,  soil,  or  manure.  Evalu- 
ating the  similarities  of  the  disease  po- 
tential of  two  organisms  is  a  specialist's 
job,  and  a  grower  can  with  safety  only 
assume  them  to  be  different  until  proved 
otherwise,  even  though  the  same  scien- 
tific or  common  name  is  applied  to  them 
or  to  the  diseases  they  cause.  Strains  of 
a  single  species  may  differ  in:  (1)  the 
hosts  which  they  can  attack  (fig.  124)  ; 
(2)  the  virulence  of  attack  (fig.  124), 
which  on  a  given  host  may  range  from 
nonpathogenic  to  highly  potent;  (3)  the 
temperatures  at  which  attack  will  occur ; 
(4)  the  ability  to  develop  in  the  lower 
levels  of  soil  and  to  withstand  appreci- 
able concentrations  of  carbon  dioxide. 
(Sec.  15.) 

Although  one  strain  of  a  fungus  may 
be  present  in  a  field  without  causing 
disease,  the  introduction  of  another 
strain  of  the  same  species  may  produce 
an  epidemic.  Sometimes  the  introduc- 
tion of  an  organism  may  even  increase 
the  loss  produced  by  a  different  one  al- 
ready present.  Prompt  severe  losses  are 
usually  sustained  when  infested  stock  is 
planted,  and  clean  soil  may  be  perman- 
ently infested,  or  the  inoculum  potential 
of  it  may  be  increased.  One  of  the  most 
dangerous  features  of  transmission  of 
organisms  with  seed  or  vegetative  parts 
is  that  the  constant  association  of  the 
virulent  strains  of  a  pathogen  and  the 
host  is  thus  assured.  A  second  serious 
factor  is  that  many  soil  organisms  will 
persist  for  many  years  in  a  soil,  once 
introduced  and  established  there.  These 
facts   all   emphasize   the    importance   of 


6  1 


eliminating  disease  organisms  in  the 
nursery,  rather  than  merely  suppressing 
or  fighting  them.   (Sec.  15.) 

Don't  fight  'em,  eliminate  'em 

Among  the  many  advantages  of  free- 
dom from  disease  in  nursery  crops  are 
the  following:  It  is  an  aid  to  easier,  more 
certain,  and  less  expensive  crop  produc- 
tion. While  growers  can  eventually  learn 
to  live  with  a  disease,  they  would  almost 
always  be  better  off  without  it,  because 
of  the  enlarged  growth  potentialities  for 
the  host.  Thus,  V erticillium  wilt  of 
chrysanthemum  can  be  controlled  by 
using  resistant  varieties,  but  more,  and 
in  some  cases  better,  varieties  are  avail- 
able if  this  restriction  is  absent.  Simi- 
larly, losses  from  Phytophthora  root  rot 
of  heather  can  be  reduced  by  minimal 
watering,  but  plant  growth  is  retarded 
and  more  skill  is  required  in  watering 
than  when  the  disease  is  absent.  (Sec.  3.) 

The  degree  of  financial  benefit  from 
disease  control  is  relative  to  the  cultural 
proficiency  of  the  grower;  the  greater 
the  return  per  plant,  the  greater  will  be 
the  profit  if  disease  in  it  is  prevented. 

Buying  or  growing  pathogen-free  nurs- 
ery stock  frequently  is  cheaper  than 
fighting  the  disease. 

The  danger  of  national  panics  in  the 
nursery  industry,  such  as  those  concern- 
ing rose  mosaic  in  1929-1932  and  chrys- 
anthemum virus  stunt  in  1947-1950, 
would  be  considerably  reduced  if  patho- 
gen-free stock  were  generally  used. 

All  nursery  practices 
must  mesh 

Disease  in  the  nursery  should  never 
be  viewed  as  an  isolated  phenomenon, 
unrelated  to  other  phases  of  growing. 
Actually  it  is  one  of  a  series  of  interre- 
lated problems  which  must  be  solved 
simultaneously  rather  than  piecemeal, 
for  maximum  effectiveness  and  perman- 
ence. A  case  in  point  is  the  perennial 
dilemma  of  southern  California  nursery- 


men, who  lose  their  seedlings  from  sa- 
linity if  they  water  lightly,  and  from 
damping-off  if  they  water  heavily.  To 
successfully  mechanize  such  a  nursery 
and  introduce  mechanical  watering,  the 
soil  pathogens  must  be  eliminated,  so 
that  the  salinity  may  be  held  down  by 
copious  watering,  without  aggravating 
damping-off.  Again,  when  soil  is 
steamed,  the  frequency  and  quantity  of 
watering  must  be  modified  because  of 
altered  water  retention.  (Sees.  2  and  3.) 

A  disease-control  program  must  either 
fit  into  the  current  cultural  methods,  or 
these  must  be  modified  before  it  can  be 
adopted.  Often  both  develop  together. 
For  example,  the  control  of  carnation 
diseases  has  developed  along  with  sweep- 
ing changes  in  methods  of  growing:  (1) 
year-round  glasshouse  culture;  (2)  di- 
rect benching  of  cuttings;  (3)  the  single 
pinch;  (4)  continuous  forcing;  (5)  use 
of  cuttings  from  the  new  disease-free 
seedlings. 

These  situations  illustrate  the  basic 
unity  in  proper  nursery  production  pro- 
cedures. If  an  advance  is  made  in  dis- 
ease control,  its  successful  adoption 
necessitates  changes  in  other  practices, 
and  its  benefits  may  project  into  other 
aspects  of  nursery  culture. 

Salinity 

Small  amounts  of  various  salts  are 
necessary  to  plant  development,  but  ex- 
cessive concentrations  cause  injury  or 
death,  just  as  small  amounts  of  table  salt 
are  necessary  to  man  but  large  amounts 
may  aggravate  high  blood  pressure  and 
very  large  amounts  may  kill. 

Plant  injury  varies 

Plant  susceptibility  to  an  excess  of 
water-soluble  salts  varies  widely  from 
almost  no  injury  (carnation,  stock;  fig 
3)  to  severe  injury  (plants  with  soft  large 
leaves,  such  as  begonia  and  fern;  figs. 
54  and  55).  Salinity  may  produce  sev- 
eral types  of  effects: 


[7] 


Fig.  3.  Salinity  injury  to  leaves  of  three  ornamentals.  A,  Maranta  leuconeura,  showing  leaf 
with  burned  tip  at  left,  normal  at  right.  B,  Apex  of  leaves  of  Cordyline  terminalis,  showing  tip- 
burn  at  left,  normal  at  right.  C,  Leaves  of  stock,  Matthiola  incana,  showing  minor  tipburn  of  old 
leaf  at  left,  and  more  serious  injury  of  young  leaf  at  right.  Such  injury  to  stock  is  important 
mainly  because  it  is  readily  infected  by  the  Botrytis  gray  mold. 


l.No  apparent  symptoms  at  moderate 
concentrations,  reduced  growth  at  higher 
levels  (carnation  I  ; 

2.  Leaf  burn,  usually  at  margins  I  azalea  I 
or   tips    (cymbidium,   Maranta,   Cordy- 


line; fig.  3),  which  makes  the  plant  un- 
sightly and  weakens  it; 

3.  Root  corrosion  and  killing  (azalea, 
gardenia;  fig.  56)  which  may,  depend- 
ing on  severity  of  injury,  cause  chlorosis 


[8] 


of  the  leaves  or  wilting  and  collapse  of 
the  plant; 

4.  Little  or  no  germination  of  seed; 

5.  Prompt  wilting,  desiccation,  and 
death  after  transplanting  seedlings  into 
saline  soil; 

6.  Localized  injury  to  leaves  from  salts 
accumulatively  deposited  on  the  surface 
by  irrigation  water  (begonia)  or  from 
contact  with  a  salt-saturated  flower  pot 
( Saintpaulia) . 

The  weakening  of  the  plant  renders 
it  more  susceptible  to  attack  by  organ- 
isms; thus  seedlings  may  be  made  more 
susceptible  to  damping-off,  and  stock 
leaves  with  tipburn  become  susceptible 
to  Botrytis  infection.  (Sec.  4.) 

Sources  of  salinity 

Salts  may  accumulate  from  those  in- 
troduced into  soil  with:  (1)  irrigation 
water  (fig.  38),  which  at  times  is  quite 
saline;  (2)  application  of  fertilizers  (fig. 
40)  in  excessive  amounts  or  of  types 
which  leave  substantial  unused  residues 
in  the  soil;  (3)  use  of  manure  or  leaf 
mold  (fig.  42)  gathered  in  places  where 
they  have  accumulated  large  amounts  of 
salt,  a  condition  common  in  southern 
California.  (Sec.  4.) 

Salts  added  in  any  of  these  ways  may: 
(1)  be  flushed  on  down  by  subsequent 
large  applications  of  water,  and  thus 
leached  beyond  the  root  zone  or  out  of 
the  container;  (2)  be  absorbed  by  plants 
and  either  used  in  their  metabolism,  or 
accumulate  and  finally  reach  a  toxic  level 
(as  in  leaf-margin  or  tipburn)  ;  (3)  ac- 
cumulate in  the  soil,  being  washed  down 
a  few  inches  with  each  irrigation,  only  to 
be  carried  back  to  the  surface  with  the 
water  and  deposited  there  as  a  residue 
when  it  evaporates.  It  is  necessary  to 
apply  more  water  than  is  used  by  the 
plants  and  evaporated  from  the  soil  sur- 
face, in  order  to  prevent  the  accumula- 
tion of  salts.  The  greater  the  salinity  of 
the  water,  the  greater  must  this  excess 
be.   Repeated   light   sprinkling  leads   to 


trouble  unless  an  occasional  heavy  leach- 
ing irrigation  is  practiced.   (Sec.  4.) 

The  salts  may  accumulate  in  a  clay 
pot  in  the  same  way,  exposing  the  roots, 
which  are  most  plentiful  next  to  it,  to  a 
high  concentration  of  salts.  (Sees.  4  and 
12.) 

Because  the  concentration  of  salts  in 
soil  water  increases  as  the  soil  dries  out, 
plant  injury  is  aggravated  by  either 
growing  plants  "on  the  dry  side"  or 
watering  only  when  the  soil  is  dry.  A 
consistently  moist  soil  will  give  least  in- 
jury.  (Sec.  4.) 

Measuring  salinity 

When  salinity  is  suspected,  because  of 
plant  injury  or  the  type  of  culture,  it  may 
be  tested  for  by  measuring  the  elec- 
trical conductance  of  a  saturated  soil 
extract  with  an  inexpensive  simple  in- 
strument, the  Solubridge.  The  salt  con- 
tent of  the  water  supply  should  be 
determined,  as  well  as  that  of  the  soil, 
peat,  leaf  mold,  manure,  and  so  on,  be- 
fore they  are  mixed  for  use.  This  elimi- 
nation of  a  source  of  potential  trouble, 
rather  than  waiting  for  it  to  develop,  is 
as  necessary  for  mechanized  nursery 
operation  as  is  the  establishment  of  qual- 
ity tolerances  for  parts  used  in  an  auto- 
mobile assembly  line.  (Sec.  4.) 

Toxicity 

Use  of  the  U.  C.  system  of  soil  mixes 
and  nutrients  has  eliminated  the  toxic 
effect  from  steaming  or  chemically  treat- 
ing soil.  Because  of  this  simple  solution, 
the  problem  seems  to  be  unnecessarily 
causing  concern.  Plants  grown  in  con- 
ventional treated  soil  mixes  may  develop 
injury  (stunting,  dropping  of  leaves, 
root  corrosion,  death )  from  toxins. 
These  materials,  which  may  be  tempo- 
rary or  persist  for  months,  result  from 
the  formation  or  release  of  various  chem- 
ical materials  (such  as  ammonium,  or- 
ganic matter,  manganese,  soluble  salts). 
Soil   mixtures   high    in    readily    decom- 


[9] 


posable  organic  matter  (manure,  leaf 
mold,  compost)  are  most  likely  to  give 
injury  from  such  treatment.  Growers 
who  do  not  use  mixes  of  the  U.  C.  type 
may  reduce  injury  by  leaching  or  aging 
the  treated  soil  before  use,  application 
of  gypsum  in  some  cases,  or  by  plant- 
ing immediately  after  treatment.  (Sec. 
6.) 


Injury  may  also  result  from  a  persist- 
ent residue  of  the  chemical  used  in  soil 
treatment  (for  example,  on  carnations 
planted  in  soil  treated  with  methyl 
bromide).  This  is  a  different  problem, 
and  may  be  prevented  by  using  a  dif- 
ferent chemical,  by  altering  the  condi- 
tions of  its  use,  or  by  aging  of  soil  after 
treatment.  (Sec.  11.) 


THE  ANSWERS 


Disease  control  in  the  nursery  is  most 
effective  when  preventive  treatment  aims 
at  eliminating  the  causal  organisms  from 
the  soil,  from  the  seeds,  cuttings,  or  other 
planting  material,  and  from  the  bits  of 
soil  on  tools,  flats,  hoses,  and  other  equip- 
ment, or  in  places  where  it  may  be 
readily  splashed  by  water  (fig.  4).  This 
approach  demands  that  preventive  meas- 
ures be  planned  in  advance,  rather  than 
waiting  until  trouble  arises.  (Sec.  3.) 

The  U.C.  System  of  Soil  Mixes 
and  Nutrients 

One  of  the  commonest  erroneous  ideas 
in  nursery  practice  is  that  a  special  soil 
is  required  for  each  type  of  plant.  Actu- 
ally, most  plants  of  necessity  had  to  have 
a  wide  tolerance  of  different  soils  in 
order  to  survive.  The  basic  fact  that 
many  kinds  of  plants  can  be  successfully 
grown  in  a  single  soil  mix,  or  in  slight 
modifications  of  it,  was  demonstrated  by 
the  John  Innes  Horticultural  Institution 
in  1934-1939.  This  was  an  important 
contribution  to  nursery  practice,  and  the 
J.  I.  mixes  have  been  widely  adopted  in 
recognition  of  this  fact.  They  fail,  how- 
ever, to  eliminate  several  serious  inher- 
ent disadvantages  common  also  to 
conventional  soil  mixes.  The  U.  C.-type 
soil  mixes  have  corrected  these  objec- 
tions. (Sec.  6.) 


Advantages  of  the  U.  C.-type 
soil  mixes 

The  soil  serves  four  principal  func- 
tions for  plants:  (1)  it  provides  me- 
chanical anchorage  and  support;  (2)  it 
stores  and  makes  available  a  supply  of 
water;  (3)  it  stores  and  regularly  sup- 
plies mineral  salts  essential  to  the  plant; 
(4)  it  provides  aeration  for  the  roots. 
These  functions  are  well  served  by  the 
U.  C.  soil  mixes.  (Sec.  6.) 

Certain  advantages  over  the  multi- 
plicity of  mixes  are  also  provided  by 
the  U.  C.-type  mixes: 

1.  They    may    be    heat-    or    chemically 

treated  without  producing  injurious 
toxic  residues. 

2.  The  variability  from  the  use  of  leaf 

mold,  animal  manure,  turf,  and 
composts,  as  well  as  from  differences 
in  the  degree  of  their  decomposi- 
tion, is  reduced,  and  more  uniform 
results  are  possible. 

3.  The   salinity   problem   is   reduced   by 

eliminating  some  of  its  common 
sources  and  by  providing  a  medium 
that  may  be  readily  and  effectively 
leached. 

4.  Labor  requirement  is  reduced. 

5.  The  space  utilized  for  compost  piles 

and  storage  of  raw  materials  and 


Fig.  4.  Diagram  of  nursery  soil  problems  considered  in  this  manual,  showing  their  sources  and 
answers.  The  numbered  boxes  on  the  source  lines  indicate  the  appropriate  preventive  measures, 
named  at  the  right. 


I  10] 


SOIL 

P 

R  O 

6 

L 

EMS 

\                   D 

T 

s              / 

\                   1 

O 

A                 / 

\                 s 

X 

L                 / 

\                E 

1 

1                / 

\              A 
\             s 

c 
1 

N              / 
1               / 

\             E 

T 

T             / 

V           s 

Y 

Y            / 

1 

/ 

s 
o 
u 

R 

c 

E 

S 


10 


Fertilizer 


Organic  Matter  — 
Planting  Stock 
Containers 
Recontamination 


ANSWERS 

1.  U.  C.  Type  Soil  Mix 

2.  Good  drainage 

3.  Moist  soil  and  air 

4.  Leaching 

5.  Treating  soil 

6.  Good  water 

7.  Frequent  light  fertilizing 

8.  Clean  planting  stock 

9.  Treating  container 
10.  Sanitation 


[in 


various  mixes  is  saved,  an  import- 
ant item  where  land  is  expensive. 

6.  Loss  of  volume  from  shrinkage  during 

composting    of    organic    matter    is 
avoided. 

7.  Odors  and  flies  from  the  compost  piles, 

likely  to   prove  restrictive   in   resi- 
dential areas,  are  eliminated. 

8.  The  problem   of  the  scarcity  of  leaf 

mold,   animal  manure,   and  turf  is 
avoided.  (Sec.  6.) 

Physical  components 

The  physical  base  of  the  U.  C.-type 
mix  consists  of  an  inorganic  material 
(fine  sand,  perlite,  vermiculite)  and  an 
organic  fraction  (sphagnum  peat  moss, 
rice  hulls,  sawdust,  shavings,  bark).  The 
two  components  presently  suggested  for 
use  in  California  nurseries  are  fine  sand 
(particle  size  ranging  from  0.5  to  0.05 
mm)  and  sphagnum  peat  moss.  These 
components  satisfy  the  greatest  number 
of  desirable  features  (fig.  65)  : 

1.  They  are  readily  available  in  uniform 

grade. 

2.  They    are    chemically    uniform    and 

relatively  inert. 

3.  They  are  not  broken  down  by  steam 

or  chemical  treatments  used  to  free 
them  of  disease  organisms. 

4.  They  are  easily  made  into  a  uniform 

mix. 

5.  They  provide  good  aeration  and  wa- 

ter drainage. 

6.  The   peat   retains   mineral   nutrients 

against  leaching,  although  the  fine 
sand  is  less  effective  in  this. 

7.  Their   fertility   is  low,   furnishing  a 

known  low  starting  point  for  add- 
ing nutrients. 

8.  They  are  relatively  inexpensive. 

().  When   mixed   they   have  good  water 
retention. 

10.  They  arc  light  in  weight. 


11.  They  have  proved  adequate  in  micro- 

nutrients,  but  should  instances  of 
deficiency  arise  the  elements  can 
easily  be  added. 

12.  They   have   negligible    shrinkage    in 

storage  and  use. 

Characteristics  1  through  4  are  essen- 
tial, and  their  deficiency  cannot  be  made 
up  by  adding  or  substituting  other  mate- 
rials, as  can  features  5  through  12.  This 
fine-sand— peat  mixture  has  most  of  the 
good  features  of  clay  soils  without  their 
disadvantages.  (Sec.  6.) 

The  sand  ingredient  may  have  12  to 
15  per  cent  (preferably  less)  coarse 
sand,  must  not  have  more  than  15  per 
cent  (preferably  less)  clay  or  silt,  or 
both,  and  should  have  70  to  85  per 
cent  or  more  of  fine  sand.  A  method  is 
available  for  nurserymen  to  determine 
particle  size  of  soil  samples.  Five  dif- 
ferent proportions  of  sand  to  peat  are 
suggested  for  different  purposes: 

Mix  A.  4:0,  sometimes  used  for  certain 
crops  and  bench  stocks; 

Mix  B.  3:1,  the  most  commonly  used 
ratio,  for  bedding  plants  and  general 
nursery  planting; 

Mix  C.  2:2,  for  plants  grown  in  pots  or 
benches; 

Mix  D.  1:3,  for  pot  plants  that  are  large 
in  relation  to  their  containers,  and 
for  cymbidiums; 

Mix  E.  0:4,  for  growing  azaleas  and 
similar  acid  plants,  sometimes 
mixed  with  wood  shavings.  (Sec.  5.) 

Chemical  components 

These  U.  C.  mixes  are  purposely  low  in 
nutrients,  so  that  mineral  elements  may 
be  added  to  this  known  base  with  pre- 
dictable results.  Sometimes  fertilizers 
are  omitted  from  the  mix  and  applied 
in  solution  after  planting.  Usually,  how- 
ever, they  are  added,  at  least  in  part,  at 
the  time  of  mixing  the  soil.  Phosphorus, 
because  it  is  applied  as  a  slowly  soluble 


12  | 


NOTE: 

Urea  and  urea-formaldehyde  fertilizers 
may  contain  biuret,  a  by-product  toxic 
to  many  plants.  Unless  labeled  biuret- 
free,  these  materials  should  be  used  only 
after  thorough  testing  on  each  crop. 


superphosphate,  does  not  contribute  ap- 
preciably to  the  salinity  problem  while 
supplying  crop  needs  for  this  element. 
Potassium  is  presently  supplied  as  water- 
soluble  potassium  nitrate  or,  less  com- 
monly, as  potassium  sulfate  or  potassium 
chloride,  and  thus  contributes  to  the 
salinity  problem.  Calcium  and  magne- 
sium are  presently  supplied  as  dolomite 
lime  in  the  soil  mix.  This  does  not  con- 
tribute appreciably  to  the  salinity  prob- 
lem, while  neutralizing  the  acidity  of  the 
peat  moss  and  supplying  these  necessary 
elements.  (Sec.  5.) 

Nitrogen  is  generally  supplied  in  the 
organic  form  (hoof  and  horn  meal,  urea- 
formaldehyde  resin,  blood  meal,  cotton- 
seed meal,  castor-bean  pomace),  but 
sometimes  as  inorganic  nitrogen  (po- 
tassium or  calcium  nitrate),  or  both. 
(Sec.  5.) 

Nitrogen  availability 

The  organic  nitrogen,  which  is  un- 
available to  the  plants,  is  converted  to 
ammonium  (available  to  plants)  by  bac- 
teria and  fungi  that  are  quite  resistant 
to  soil  steaming  or  chemical  treatment; 
the  ammonium  is  changed  to  nitrate 
(available  to  plants)  by  nitrifying  bac- 
teria intolerant  of  most  of  these  treat- 
ments. The  latter  step  is  adversely 
affected  by  an  acid  medium  and  low  soil 
temperature,  and  therefore  ammonium 
tends  to  accumulate  under  these  condi- 
tions. Also,  bacteria  tend  to  be  sparse  in 
fine  sand  obtained  at  a  depth  of  a  foot 
or  more  (as  much  of  it  is  in  California) , 
and  in  this  way  such  material  resembles 
treated  soil.  (Sees.  6,  7,  and  14.) 

Under  average  conditions  treated  soil 


containing  organic  nitrogen  does  not,  for 
a  variable  period  (about  1  week),  pro- 
vide nitrogen  in  a  form  available  to 
plants,  then  presents  it  in  the  form  of 
ammonium  for  another  period  (about  2 
weeks),  and  thereafter  as  both  am- 
monium and  nitrate.  Seedlings  grown  in 
a  newly  treated  soil  containing  only 
organic  nitrogen  are  often  deficient  in 
nitrogen,  a  situation  readily  corrected  by 
watering  with  a  calcium  nitrate  starter 
solution  at  time  of  planting.  (Sec.  7.) 

Nursery  crops  differ  in  their  response 
to  ammonium.  Many  types  of  crops  (for 
example,  foliage  plants)  utilize  the  am- 
monium nitrogen  without  ill  effect,  and 
these  are  successfully  grown  with  the 
organic  source.  Other  plants  (for  ex- 
ample, sweet  alyssum,  clarkia,  and  car- 
nation) show  injuries  varying  from 
leaf  burn  and  root  injury  to  death,  when 
supplied  ammonium  nitrogen.  Petunia 
and  snapdragon  seedlings  may  show  ex- 
cessively soft  growth  and  iron  chlorosis. 
This  injury  is  worst  to  plants  in  the 
seedling  stage.  In  such  cases  the  organic 
nitrogen  should  be  eliminated  or  reduced 
in  amount,  and  nitrate  fertilizer  should 
be  supplied.  (Sees.  5,  7,  and  14.) 

The  quantity  of  organic  nitrogen  sup- 
plied varies  with  the  size  of  the  plant  in 
relation  to  the  volume  of  soil  in  the  con- 
tainer, and  is  greatest  for  large  plants  in 
small  containers. 

It  may  be  possible  to  inoculate  treated 
soil  with  nitrifying  bacteria  in  order  to 
lessen  the  ammonium  increase,  but  the 
effect  may  not  be  evident  for  10  to  20 
days.  If  this  is  done,  nitrifiers  should  be 
introduced  without  ammonifiers,  or  the 
situation  may  be  worsened.  (Sees.  5,  7, 
and  14.) 

Because  the  breakdown  of  organic 
nitrogen  occurs  in  a  stored  U.  C.-type 
mix,  the  high  content  of  water-soluble 
forms  of  nitrogen  may  cause  plant  injury 
if  the  soil  is  held  for  several  weeks  be- 
tween mixing  and  planting.  If  such 
storage  is  planned,  one  of  the  fertilizer 
variants   should   be   used   that   does   not 


[13] 


include  the  organic  form  of  nitrogen; 
this  form  should  be  applied  as  a  top 
dressing  after  planting.  (Sees.  5,  7,  and 
14.) 

Selecting  nitrogen  fertilizers 

The  organic  forms  are  slowly  available 
and,  therefore,  present  a  continuous  low 
supply  rather  than  the  varying  quantities 
resulting  from  occasional  applications  of 
inorganic  forms.  In  descending  order  for 
rate  of  conversion  and  ascending  order 
for  nitrogen  content,  the  organic  nitro- 
gens tested  were:  castor  pomace;  fish 
meal;  cottonseed  meal;  blood  meal  and 
hoof  and  horn  meal;  urea-formaldehyde 
resins.  The  urea-formaldehyde  resins 
are  apparently  hydrolyzed  by  steaming 
and  may  therefore  give  rapid  ammonium 
build-up;  these  materials  must  be  free  of 
biuret  to  be  safe  for  use  on  nursery 
plants.  Hoof  and  horn  meal  and  blood 
meal  are  presently  considered  best  for 
the  mix.   (Sees.  5  and  7.) 

Surface  dressings  of  these  organic  ma- 
terials do  not  lead  to  injury  from  am- 
monium, because  of  the  slow  rate  of 
penetration  of  ammonium  through  soil; 
it  is  normally  converted  to  nitrate  before 
reaching  the  root  zone.  (Sees.  5  and  7.) 

The  nitrate  forms  are  immediately 
available  and  eliminate  the  hazard  of 
ammonium  accumulation.  They  are 
therefore  useful  (1)  in  mixes  when  nitri- 
fying bacteria  are  absent  (sees.  7  and 
14)  ;  (2)  in  starter  solutions  for  seed- 
lings in  treated  mixes  containing  organic 
nitrogen  (Sec.  7)  ;  (3)  when  soil  mixes 
are  to  be  stored   (Sec.  5). 

Fertilizer  application 

Where  plants  are  to  be  carried  in 
containers  or  benches  for  an  extended 
period  of  time,  it  becomes  necessary  to 
replace  fertilizers  which  are  lost  through 
plant  uptake  and  leaching.  Dry  fertilizer 
may  be  broadcast  over  the  surface  and 
watered  in,  or  liquids  may  be  applied  in 
the  irrigation  water  itself.  (Sec.  5.) 

I  sually  dry  fertilizers  should  include 


an  organic  source  of  nitrogen,  in  order 
to  prolong  the  effective  period  and  reduce 
the  salinity  hazard.  There  is  no  problem 
with  ammonium  from  such  application, 
even  though  it  may  be  formed.  Am- 
monium does  not  move  readily  down 
past  the  surface  layer  of  soil  until  broken 
down  by  microorganisms  to  nitrate.  The 
other  components  of  the  fertilizer  should 
be  superphosphate  and  potassium  sulfate. 
(Sees.  5  and  7.) 

Liquid  fertilizers  may  contain  any  one 
of  several  nitrogen  sources.  Ammonium 
nitrate  or  urea  is  commonly  used,  as 
they  can  readily  be  mixed  with  any  other 
ingredients.  Calcium  nitrate  should  be 
used  where  an  all-nitrate  form  is  desired, 
but  this  source  of  nitrogen  should  not  be 
mixed  with  sulfates  or  phosphates  in  con- 
centrates because  it  forms  insoluble  salts 
of  calcium  phosphate  and  calcium  sul- 
fate. Phosphate  is  easily  supplied  as 
mono-ammonium  phosphate,  and  potas- 
sium as  potassium  chloride.  Other  mate- 
rials can  be  used,  and  examples  of  rates 
and  formulas  are  outlined.  (Sees.  5  and 
7.) 

Preventing  Salinity  Injury 

Excessive  concentrations  of  salts  may 
be  avoided  through  detection  of  accumu- 
lation at  an  early  stage  by  measuring  the 
electrical  conductance  of  soil  samples, 
and  taking  active  preventive  measures. 
Once  the  plant  is  crippled  by  root  injury 
or  leaf  burning,  rehabilitation  is  slow, 
and  seldom  economic  for  a  nursery 
crop.  Preventive  measures  include: 

1.  Use  excess  water  over  that  required 

for  plant  absorption  and  evapora- 
tion from  the  soil,  assuring  drainage 
from  the  root  zone  with  each  water- 
ing. 

2.  Use  the  best  possible  quality  of  water 

(low  salinity;  conductance  less  than 
1.0,  or  650  ppm),  and  avoid  saline 
waters,  particularly  on  sensitive 
plants. 

3.  Leach  with  a  considerable  excess  of 

water  whenever  salts  reach  a  dan- 


I  14] 


gerous  level  (conductance  of  satura- 
tion extract  about  3.0,  or  1,950  ppm 
for  most  crops)  ;  the  higher  the 
salinity  of  the  water  or  soil  the 
greater  the  quantity  of  water  that 
must  be  used. 

4.  Use  deionized  water  for  some  of  the 

more  expensive  crops. 

5.  Expose    soil   and   plants   to    rainfall 

(salt-free)   when  feasible. 

6.  Maintain   excellent  drainage    (unob- 

structed drainage  holes  in  pots  and 
cans;  open  cracks  in  bottoms  of 
benches;  avoid  hardpan  soils  below 
ground  beds;  install  drain  tiles 
when  needed;  use  porous  soil)  so 
that  salts  may  be  readily  flushed 
from  the  soil. 

7.  Use  a  U.  C.-type  soil  mix  with  the 

organic  matter  in  small  pieces; 
avoid  loam  (even  if  Krillium- 
treated) ,  as  it  does  not  leach  readily. 

8.  Avoid  soil,  manure,  leaf  mold,  black 

peat,  and  kinds  of  sewage  sludge 
high  in  soluble  salts  or,  if  they  must 
be  used,  leach  them  heavily  before- 
hand. 

9.  Apply    fertilizer    in    frequent   small, 

rather  than  in  a  few  heavy,  applica- 
tions. 

10.  Avoid  mixed  fertilizers  that  include 

minerals  not  needed  at  all  or  not  in 
amounts  supplied. 

11.  Grow  plants  with  the  greatest  shade 

and  humidity  compatible  with  good 
culture  of  the  crop,  so  as  to  decrease 
salt  concentration  and  injury. 

12.  Keep  soil  as  uniformly  moist  as  is 

compatible  with  good  culture  of  the 
crop,  avoiding  alternating  wet  and 
dry  soil  conditions. 

13.  Add   organic   matter   to   the  soil   to 

stabilize  moisture  content  and  reduce 
injury. 

14.  Soak  old  clay  pots  before  re-use  to 

remove  accumulated  salts. 


15.  Avoid  overhead  sprinkling  of  plants 
(begonia)  subject  to  leaf  burn  from 
salt    left    from    evaporated    water 
drops. 
The  U.  C.  system  of  soil  mixes  is  com- 
pletely compatible  with  these  measures, 
either  incorporating  them  or  making  it 
possible  to  follow  the  procedures.   (Sec. 
4.) 

Treatment  of  Soil  by  Heat 

Heat  treatment  is  used  to  free  soil  of 
organisms  which  cause  plant  disease,  as 
well  as  of  weed  seeds  and  insects.  Steam 
treatment  of  soil  remains  the  best  method 
of  disinfestation  for  all  fungi,  bacteria, 
nematodes,  weed  seed,  and  insects.  (Sees. 
8  and  9.) 

Temperature  requirements 
and  soil  preparation 

Heat  treatments  in  which  a  temperature 
of  180°  F  is  maintained  for  30  minutes 
are  adequate;  with  many  methods,  how- 
ever, the  process  cannot  be  stopped  short 
of  212°.  With  the  U.  C.  system  of  soil 
mixes  there  is  no  residual  toxicity  from 
heating  in  either  case.  The  soil  should  be 
in  good  planting  tilth,  well  mixed,  free 
of  clods,  and  with  sufficient  moisture  so 
that  after  being  squeezed  in  the  hand  it 
will  crumble  easily.  For  economical 
treatment,  the  soil  should  not  be  soggy, 
because  five  times  more  heat  is  required 
to  heat  water  than  soil.  After  cooling,  the 
soil  is  in  good  planting  condition,  with- 
out excessive  moisture.  (Sec.  8.) 

Methods 

Treatments  are  preferably  applied 
after  the  soil  is  mixed  and  placed  in  tlie 
containers,  since  this  method  reduces  the 
recontamination  hazard  from  handling. 
Some  of  the  bulk  methods  of  soil  treat- 
ment are,  however,  satisfactory  if  mech- 
anized to  minimize  the  handling.  (Sees. 
8,  9,  and  10.) 

Steam  is  most  economically  used  in 
the  free-flowing  form.  When  steam  is 
released    into    soil    under    pressure    it 


[15] 


immediately  reverts  to  212°  F  and  the 
nonpressure  (relative  to  atmospheric 
pressure)  condition;  therefore,  there  is 
little  advantage  in  using  it.  Since  super- 
heated steam  provides  a  little  more  heat 
per  pound  and  contains  somewhat  less 
entrained  water,  there  are  some  slight 
advantages  in  using  it,  although  equip- 
ment of  this  type  is  not  commonly 
available  in  this  country.  (Sees.  8  and  9. ) 
The  quantity  of  steam  needed  to  raise 
the  soil  temperature  to  212°  F  varies  with 
many  factors,  but  a  generally  accepted 
working  average  is  6.5  pounds  per  cubic 
foot  of  soil,  or  42  B.t.u.  per  cubic  foot 
per  degree  of  rise.  The  amount  of  soil 
that  can  be  steamed  in  a  given  time  using 
boilers  of  various  sizes  is  given  in  table 
14.  One  of  the  advantages  of  steam  for 
soil  treatment  is  that  it  may  be  used  near 
living  plants  without  injuring  them;  it 
is  neither  toxic  nor  unpleasant  to  work- 
men. (Sees.  8  and  9.) 

Equipment 

Many  kinds  of  equipment  have  been 
designed  for  steaming  soil,  and  these 
have  been  grouped  in  this  manual  into 
thirty-five  types.  Nine  of  these  (types  2, 
4a,  4b,  5,  6,  7,  18,  19,  and  29)  seem  well 
adapted  to  California  conditions.  (Sec. 
10.) 

For  treatment  of  bulk  soil,  stationary 
or  mobile  steam  boxes  (type  4,  fig.  5)  or 
the  mobile  bin  (type  2,  fig.  80),  both 
with  perforated  pipe  grid  and  a  sta- 
tionary soil  mass,  are  excellent.  The 
feature  of  continuous  output  may  be 
combined  with  the  advantages  of  having 
a  stationary  soil  mass  in  the  continuous- 
batch  modification  of  types  2,  4,  6,  or  9 
(fig.  6) .  For  a  continuous  output  of  bulk 
soil,  the  rotating-screw  type  with  injected 
steam  (type  29,  fig.  100)  is  satisfactory. 
For  treatment  of  soil  in  containers,  the 
Thomas  method  (type  5),  the  vault  (type 
0.  fig.  L31),  and  the  multipurpose  tank 
(type  7,  fig.  85)  arc  recommended.  The 
Thomas  method  (type  L8,  fig.  ()\  I  and 
the  inverted  steam  pan  (type  19.  fin.  92  I 


are  the  most  convenient  for  treating  soil 
in  benches  and  beds,  but  are  not  depend- 
able below  8  to  9  inches  in  depth.  If  deep- 
er treatment  is  required,  the  buried  per- 
forated pipe  (type  20,  fig.  93)  or  the 
permanent  buried  tile  (type  22,  fig.  95) 
may  be  used.  (Sec.  10.) 

Equipment  in  which  a  stationary  soil 
mass  is  treated  raises  the  temperature  to 
212°  F  when  steam  is  used,  and  even 
higher  if  electric  immersion  heaters  are 
employed.  When  steam  is  used,  equip- 
ment with  a  stationary  soil  mass  is  prob- 
ably best,  although  some  with  a  moving 
soil  mass  are  satisfactory.  When  heat 
from  immersion  units,  hot  plates,  and 
other  dry  sources  of  heat  is  used,  it  is  de- 
sirable that  a  type  with  a  moving  soil 
mass  be  employed,  since  there  is  charring 
of  organic  matter  in  types  with  a  station- 
ary soil  mass.  The  use  of  hot  water  for 
soil  treatment  is  suitable  only  for  propa- 
gating sand.  One  should  use  table  15  to 
determine  the  types  of  equipment  with 
the  necessary  features  for  a  given  instal- 
lation, and  then  refer  to  the  text  for 
details.  The  size  of  the  boiler  required 
may  be  estimated  from  table  14.  (Sees. 
9  and  10.) 

Natural  gas  is  the  cheapest  fuel  in 
California.  Oil  and  butane  are  more  ex- 
pensive but  better  adapted  to  portable 
equipment.  Electricity  is  very  expensive 
to  use  for  soil  treatment,  but  is  very 
convenient.  (Sec.  10.) 

Steam  versus  chemicals 

Steaming  soil  requires  about  an  hour, 
plus  another  hour  to  cool  before  plant- 
ing; methyl  bromide  is  used  in  a  24-  to 
48-hour  treatment,  plus  a  24-  to  48-hour 
aeration;  chloropicrin  is  used  in  a  48- 
to  72-hour  treatment,  plus  a  7-  to  10-day 
aeration.  Steam  is  effective  against  all 
organisms  except  a  few  types  of  weed 
seeds;  methyl  bromide  is  only  partially 
effective  against  Verticillium,  and  leaves 
a  residue  toxic  to  some  plants  (carna- 
tions; snapdragon  seedlings);  chloro- 
picrin    is     generally     effective     against 


[10] 


Fig.  5.  The  removable-front  steam  box  for  stationary  bulk  soil,  and  for  soil  in  containers 

(Sec.  10,  type  4b).  See  also  fig.  82. 


Fig.  6.  Pressure  autoclave  for  treatment  of  flats  of  soil  (Sec.  10,  type  9).  The  flats  are 
placed  in  a  special  rack  and  rolled  into  the  unit  for  steaming.  (Photo  courtesy  of  American 
Plant  Growers,  Lomita,  California.)  See  also  fig.  86. 


[17] 


organisms  except  those  in  root  masses, 
and  leaves  no  toxic  residue  unless  im- 
properly used.  The  cost  of  treatment 
with  steam  (including  cost  of  boiler)  is 
less  than  2.0  cents  per  cubic  foot;  for 
methyl  bromide  it  is  about  2.9  to  3.2 
cents,  and  for  chloropicrin  about  1.9  to 
3.0  cents;  labor  is  excluded  in  these  cal- 
culations. (Sec.  8.) 

Regardless  of  the  method  of  treatment, 
if  clean  soil  is  dumped  in  bulk  piles  on 
the  floor,  the  surface  should  previously 
have  been  wet  down  with  a  formaldehyde 
solution  (1  gal.  to  18  gal.  water).  (Sees. 
8  and  11.) 

Treatment  of  Soil 
by   Chemicals 

When  a  source  of  steam  is  not  avail- 
able, or  when  an  area  of  field  soil  is  to 
be  treated,  chemical  applications  are 
often  used.  Fungi  are  harder  to  kill  in 
the  soil  with  chemicals  than  are  nema- 
todes, insects,  or  weed  seeds.  When  soil 
fungi  are  involved,  the  chemical  and 
dosage  must  be  adequate  to  kill  them; 
this  dosage  will  usually  also  kill  the 
nematodes,  insects,  and  weed  seed. 
Nematocides  (DD,  EDB)  often  have 
little  fungicidal  value,  and  insecticides, 
weed  killers,  and  soil  conditioners  are 
also  largely  ineffective  against  fungi  or 
bacteria.   (Sec.  11.) 

Methyl  bromide 

Nursery  soil  in  containers  is  commonly 
treated  with  gaseous  methyl  bromide  in 
California.  The  stacked  flats,  pots,  cans, 
or  small  piles  of  soil  are  covered  with  a 
tight  plastic  tarpaulin,  and  the  gas  re- 
leased under  it  from  pressurized  cans  or 
cylinders  (figs.  106  and  107)  at  the  rate 
of  4  pounds  per  100  cubic  feet  of  en- 
rlosrd  space  If  tin1  temperatures  are 
low,  the  gas  is  passed  through  a  copper 
pipe  immersed  in  hot  water  to  volatilize 

any  Liquid  methyl  bromide.  The  soil  is 
left  covered  for  21  to  18  hours,  and 
aerated    for   24   to   48    hours    before   use. 


This  is  an  effective  and  commonly  used 
treatment,  but  should  not  be  applied  to 
soil  to  be  used  for  carnations  or  snap- 
dragon seedlings  because  of  residual 
toxicity,  or  for  chrysanthemums  because 
of  its  ineffectiveness  against  Verticillium. 
(Sec.  11.) 

Chloropicrin 

Chloropicrin  is  more  generally  useful 
and  cheaper,  but  because  it  is  less  con- 
venient and  takes  longer  to  aerate,  it  is 
less  commonly  used  in  California.  It  has 
not  been  commonly  used  for  treating 
stacked  containers  of  soil.  On  beds  it  is 
injected  6  inches  deep  with  special 
equipment  (fig.  105),  at  the  rate  of  2  to 
3  cc  per  10  inches  square  of  soil,  and  is 
confined  either  by  wetting  the  top  inch 
of  soil  or  by  covering  it  with  a  plastic 
tarp.  Bulk  soil  may  be  treated  in  bins, 
drums,  or  any  gasproof  receptacle  at  the 
rate  of  3  to  5  cc  per  cubic  foot.  Treat- 
ments with  chloropicrin  are  for  48  to  72 
hours,  plus  a  7-  to  10-day  aeration  before 
planting.  (Sec.  11.) 

Vapam 

This  new  water-soluble  material  is  be- 
coming widely  used,  especially  for 
ground  beds  and  field  soil.  It  is  applied 
at  1  to  2  quarts  per  100  square  feet, 
either  on  the  surface  with  irrigation 
water  or  injected  into  the  soil.  Soil  may 
be  cultivated  after  7,  planted  after  14 
days.  (Sec.  11.) 

Nematocides 

DD  is  used  for  nematode  control  in  the 
field  at  200  to  400  pounds  per  acre,  and 
the  soil  may  be  planted  after  1  to  2 
weeks.  Ethylene  dibromide  (EDB)  is 
used  against  nematodes  at  the  rate  of  3 
to  (>  gallons  per  acre,  and  soil  may  be 
pi  an  led  after  2  to  3  weeks.  Neither  of 
these  last  two  materials  is  recommended 
against  fungi  or  bacteria.    (Sec.  11.) 


[18] 


Spot  treatments 

Spot  treatment  of  limited  areas  of 
fungus  infection  is  desirable  to  prevent 
spread  to  other  plants,  but  it  should 
never  be  made  the  complete  control  pro- 
gram, as  it  is  in  some  nurseries.  Such 
spot  treatments  are  not  eradicative  and 
merely  aim  to  make  the  best  of  a  bad 
situation.  The  disease  suppression  af- 
forded is  temporary  and  in  many  cases 
not  very  satisfactory.  Materials  used  in- 
clude ferbam,  thiram,  and  captan  (1 
tbsp.  per  gal.  of  water,  per  8  to  16  sq. 
ft.),  Semesan  (1  tbsp.  per  gal.  of  water, 
per  24  sq.  ft.),  nabam  (1  fl.  oz.  per  4  to 
8  gal.  of  water,  per  64  sq.  ft.),  and  Ter- 
raclor  (1  oz.  of  75  per  cent  wettable 
powder  per  42  to  63  sq.  ft.).  In  treat- 
ment of  flats  the  whole  surface  should  be 
drenched;  in  the  field  the  removal  of  dis- 
eased plants  may  be  desirable  and  the 
drench  should  be  applied  to  an  area  1  to 
2  feet  beyond  the  margin  of  disease. 
(Sec.  11.) 

Treatment  of  Containers 

Flats,  pots,  cans,  benches,  and  pallets 
should  be  treated  with  heat  or  chemicals 
before  being  re-used.  Preferably  the  raw 
soil  mix  is  placed  in  the  container  and 
both  are  treated  together,  but  treating  the 
containers  and  soil  separately  is  often 
practiced. 

Heat 

If  steam  is  used  for  treatment  of  con- 
tainers, a  temperature  of  at  least  180°  F 
should  be  maintained  for  30  minutes,  the 
same  as  for  soil.  With  clay  pots  perhaps 
the  best  method  is  to  soak  thern  in  water 
of  180°-212°  for  at  least  30  minutes, 
since  this  removes  the  accumulated  salts 
and  kills  pathogens  and  algae  in  the  sur- 
face slime  as  well.  The  prevalent  practice 
of  going  over  a  bench  with  a  blow  torch 
is  without  disinfesting  value;  heat  of  this 
type  is  so  concentrated  as  to  char  the 
wood,  but  so  short  that  heat  does  not 
penetrate.    Steam    provides    the    steady 


penetrating  heat  needed  for  this  work. 
(Sec.  12.) 

Chemicals 

Chemicals  may  be  effectively  used  for 
treatment  of  containers.  Methyl  bromide 
may  be  used  for  fumigation  by  treating 
stacked  flats  exactly  as  for  soil  above; 
they  should  be  aerated  for  1  day  before 
use.  Formaldehyde  ( 1  gal.  of  37  per  cent 
commercial  formaldehyde  per  18  gal. 
water)  may  be  used  for  dipping  con- 
tainers or  may  be  sprayed  on  them,  using 
a  coarse  nozzle.  In  either  case  they  should 
be  stacked  wet  and  covered  with  a  tar- 
paulin for  24  hours  and  then  should  be 
aerated  for  at  least  4  to  5  days,  being 
kept  wet  to  prevent  the  formation  of  the 
slowly  volatilizing  paraformaldehyde. 
Tools  may  be  dipped  for  a  few  minutes 
in  a  crock  of  this  solution,  rinsed  with 
water,  and  used  at  once.  Copper  naphthe- 
nate  applied  to  flats  affords  a  self-disin- 
festing  surface  residue  for  at  least  a 
year;  there  is  no  carryover  of  damping- 
off  organisms  on  containers  treated  in 
this  way.  It  is  least  expensive  when  pur- 
chased in  the  8  per  cent  concentrate,  and 
diluted  (1  gal.  to  3  gal.  of  Stoddard 
solvent  per  800  to  1,600  sq.  ft.)  for 
dipping  or  painting  on  wood  containers. 
It  is  an  excellent  wood  preservative.  Such 
containers  may  be  steamed,  but  need  not 
be  if  they  are  freed  of  lumps  of  soil  and 
roots.  Since  there  is  some  root  injury,  the 
material  should  not  be  used  on  seed  flats 
or  benches  where  plants  are  set  less  than 
2  to  3  inches  from  the  sides.  It  is  excel- 
lent for  treated  benches,  shelves,  and 
timbers  on  which  containers  are  set,  and 
may  be  used  for  disinfesting  an  empty 
bench  in  a  house  full  of  plants,  if  the 
vents  are  kept  open  for  a  day.  (Sec.  12.) 

Development  and  Maintenance  of 
Healthy  Planting  Stock 

For  the  preceding  soil  treatments  to  be 
really  effective  it  is  necessary  that  healthy 
stock  be  planted   in  such   soil,   because 


[19] 


diseased  plants  are  centers  of  infection 
for  healthy  ones,  this  spread  of  disease 
often  being  more  rapid  in  treated  than  in 
untreated  soil.  This  unfortunate  effect 
from  treating  soil  is  actually  more  of  a 
mental  than  a  practical  obstacle  at  pres- 
ent, and  it  may  be  possible  eventually  to 
eliminate  it  through  the  use  of  antago- 
nistic or  retardant  organisms.  (Sec.  14.) 
Pathogens  introduced  into  a  planting 
are  more  dangerous  if  they  infest  the  soil 
than  if  they  do  not,  and  the  longer  they 
are  able  to  persist  in  it  the  greater  the 
danger.  Most  of  the  pathogens  with 
which  we  are  concerned  in  this  publica- 
tion more  or  less  permanently  infest  the 
soil.  (Sec.  13.) 

Obtaining  clean  stock 

Clean  stock  or  seed  may  be  initially  ob- 
tained in  the  following  ways  (Sec.  13.)  : 

1.  From    a    specialist    propagator   who 

has  maintained  it.  It  is  as  much  a 
duty  in  the  nursery  world  to  report 
to  the  propagator  any  stock  that 
carries  disease  as  it  is  to  vote  in 
the  political  world,  and  for  much 
the  same  reason. 

2.  From  a  few  healthy  plants  that  may 

be  available. 

3.  By   using   practices   which   enable   a 

few  plants  to  grow  away  from  the 
pathogen.  Tip  cuttings  produced 
12  inches  or  more  above  the  soil 
may  be  taken  from  plants  grown 
in  areas  as  nearly  free  of  disease 
microorganisms  as  possible,  and 
without  overhead  sprinkling.  Cane- 
producing  and  trailing  plants  may 
be  trained  up  off  the  ground  to 
achieve  this.  In  exceptional  cases 
aseptic  culturing  of  tiny  apical 
growing  points  may  be  used  to  pro- 
vide a  nucleus  of  healthy  stock. 
1.  By  using  cultured-cutting  techniques 
(fig.  109)  to  obtain  a  nucleus  of 
disease-free  plants.  This  has  been 
done  commercially  for  chrysanthe- 


mums, carnations,  roses,  and  gera- 
niums. It  will  not  eliminate  virus 
infections,  however.  The  technique 
involves  culturing  from  the  base  of 
each  cutting;  those  that  are  found 
to  be  clean  form  the  nucleus  for 
further  propagation.  This  involved 
technique  has  been  most  effectively 
used  by  specialist  propagators,  less 
so  by  growers. 

5.  By  heat  treatment  of  planting  stock. 

This  has  been  applied  to  a  wide 
range  of  ornamentals  including 
foliage  plants  (figs.  112  and  114), 
succulents  (fig.  Ill),  seeds,  corms, 
bulbs,  plants,  and  cane.  It  is  based 
on  the  greater  heat  sensitivity  of 
the  pathogen  than  the  host,  and 
each  treatment  is  therefore  spe- 
cific. Methods  must  be  evolved  for 
each  parasite-host  combination 
and  must  be  carefully  followed. 
The  central  idea  is  to  obtain  a 
nucleus  of  clean  stock,  and  to  do 
this  even  high  mortality  from  the 
heat  treatment  is  justified.  Actually, 
however,  there  is  very  satisfactory 
survival  in  most  cases.  The  cleanest 
most  vigorous  stocks  or  seeds 
available  should  be  used,  and  they 
should  be  in  a  state  to  withstand 
treatment  or  be  conditioned  for  it. 
The  key  to  success  in  many  cases 
lies  in  using  heat-resistant  plant 
material.  Temperatures  range  from 
115°  to  131°  F  for  10  to  40  min- 
utes for  different  plants;  details 
are  given  in  Section  13. 

6.  By    chemical    treatment.    This    has 

limited  use  in  eradicating  an  or- 
ganism in  host  tissue  (calla  rhi- 
zomes treated  with  formaldehyde, 
mercuric  chloride,  or  New  Im- 
proved Ceresan ;  chrysanthemum 
sprayed  with  parathion  against 
foliar  nematode),  for  purposes 
here  considered.  It  is  primarily 
useful   for   protective  purposes,  to 


20 


prevent  an  organism  from  invad- 
ing the  coated  tissue,  especially 
with  seeds. 

7.  By  sanitary  practices.  Though  rarely 

capable  of  providing  clean  stock, 
these  will  do  so  in  the  cases  of 
camellia  and  azalea  flower  blight 
through  elimination  of  carryover 
sclerotia. 

8.  By    aging   of   seed.    This    sometimes 

frees  the  seed  of  an  organism; 
celery  seed  3  years  or  more  old  is 
thus  freed  of  Septoria  late  blight. 

9.  By    prolonged    roguing    of    diseased 

plants  from  a  stock.  This  will 
gradually  free  it  of  a  disease  that 
does  not  spread  in  the  field  (for 
example,  rose  yellow  mosaic  in  a 
mother  block). 

10.  By   growing   plants   from   true   seed 

rather  than  from  vegetative  parts. 
This  is  particularly  effective 
against  viruses  (ranunculus,  free- 
sia,  anemone,  yellow  calla). 

11.  By  selecting  areas  for  producing  seed 

or  propagative  material  where  the 
climate  exerts  a  naturally  restric- 
tive effect  on  the  organism  in  ques- 
tion. This  will  help  in  obtaining 
pathogen-free  stock.  (Sec.  13.) 

Maintaining  clean  stock 

The  maintenance  of  the  healthy  status 
of  a  stock  must  involve  certain  safe- 
guards. 

1.  New  stock  brought  in  should  be  iso- 

lated in  an  introduction  "pest 
house"  to  determine  its  state  of 
health  before  it  is  planted  in  clean 
production  houses. 

2.  Propagation  operations  should  be  iso- 

lated from  all  possible  sources  of  in- 
fection; they  should  not  be  con- 
ducted in  weedy  areas  or  known  dis- 
ease centers. 

3.  The  production  and  merchandising  of 

plants  on  the  one  hand,  and  the 
maintenance  of  the  basic  stock  for 


future  propagation  on  the  other, 
must  be  handled  as  independent  and 
isolated  activities  (fig.  115).  This 
mother-block  principle  is  effective 
because  it  is  easier  to  care  for  and 
protect  a  small  block  than  the  large 
production  areas.  It  is  a  practice 
worthy  of  far  greater  adoption  by 
the  nursery  industry. 

4.  The  sanitary  procedures  outlined  in 
sections  3  and  14  should  be  care- 
fully followed.  (Sec.  13.) 

Preventing  Recontamination 

The  natural  soil  contains  a  vast  popu- 
lation of  roots  of  higher  plants,  algae, 
fungi,  bacteria,  actinomycetes,  insects, 
nematodes,  protozoa,  and  other  organ- 
isms. These  exist  in  a  state  of  dynamic 
equilibrium  or  fluctuating  balance,  com- 
peting with  each  other  for  food  and 
space.  One  may  become  temporarily 
dominant  due  to  some  change  (for  ex- 
ample, a  favorable  food,  temperature, 
moisture),  but  soon  is  submerged  by  the 
parasitic,  competitive,  or  antibiotic  ac- 
tivities of  other  organisms.  (Sec.  14.) 

Parasitic  fungi  and  bacteria  generally 
have  been  weakened  for  such  competi- 
tion by  the  very  specialization  required 
for  their  attack  on  higher  plants,  and  are 
unable  long  to  withstand  the  survival 
pressure  in  the  soil.  The  more  highly  de- 
veloped the  parasitic  activity,  and  the 
more  able  the  organism  is  to  attack  a 
vigorously  growing  plant,  the  less  able 
it  usually  is  to  survive  under  natural 
competition.  When  the  host  is  present, 
competition  is  evaded  by  utilizing  the 
specialized  food  source  (living  plant) 
which  the  saprophytic  forms  cannot  at- 
tack. Specialized  parasites,  such  as  the 
geranium  leaf  spot  and  stem  rot  bac- 
terium and  the  fungus  that  causes  flower 
blight  of  azalea,  survive  for  a  relatively 
short  time  in  natural  soil  free  of  the 
hosts.  Generalized  parasites  that  only  in- 
fect weakened  plants  (the  bacterium  that 
causes  soft  rot  of  many  hosts,  and  many 


[21] 


of  the  water  molds)  remain  as  part 
of  the  soil  flora,  even  without  the  pres- 
ence of  a  host.  (Sec.  14.) 

Effect  of  eliminating  soil 
microorganisms 

The  first  organisms  to  return  after  an 
eradicatory  soil  treatment  obviously  will 
luxuriate.  If  a  crop  is  planted  in  treated 
soil  it  will  generally  grow  better  than  in 
an  untreated  soil,  because  many  soil 
microorganisms  are  largely  crop-antago- 
nistic, even  though  they  may  not  produce 
disease.  Similarly,  the  first  microorgan- 
ism which  returns  develops  abundantly, 
sometimes  visibly,  in  treated  soil.  If  this 
organism  is  capable  of  causing  disease,  a 
severe  outbreak  may  occur.  This  is  the  re- 
contamination     problem;     the     risk     is 


highest  in  the  first  week,  before  relatively 
harmless  air-borne  bacteria  and  molds 
reinfest  the  soil.  (Sees.  3  and  14.) 

If  a  soil  fungicide  is  used  at  lower  or 
considerably  higher  than  recommended 
rates,  or  if  one  is  used  that  is  effective 
against  most  organisms  but  not  some 
given  pathogen  (for  example,  methyl 
bromide  against  Verticillium) ,  the  same 
effect  may  be  attained.  Therefore,  one 
should  use  recommended  dosages  of  a 
soil  fungicide  known  to  be  adequate  for 
the  job.  (Sees.  11  and  14.) 

The  common  potential  sources  of  re- 
contamination  in  the  nursery  have  been 
outlined  above  ("The  Problems — Dis- 
eases") and  in  Section  3.  The  preventive 
procedures  are  summarized  below  in  "A 
Nursery  Sanitation  Code." 


A  Nursery  Sanitation  Code3 

Treat  all  soil  used  with  either  steam  or 
chemicals.  The  chance  of  recontamination 
will  be  reduced  if  all  soil  in  a  given  glass- 
house area  is  treated.  (Sees.  8  and  11.) 

Segregate  the  clean  treated  pots  and 
flats  in  definite  areas  of  the  workroom,  iso- 
lated from  used  untreated  containers.  Don't 
place  an  untreated  pot  or  flat  in  a  pile  of 
clean  containers.  Never  store  clean  ones 
on  the  ground.  (Sec.  3.) 

Treat  floors  with  formaldehyde  solution 
before  dumping  treated  soil  on  them.  Pal- 
lets for  flats  and  pots  should  also  be 
treated.  (Sees.  8,  11,  and  12.) 

Don't  plant  clean  seed  or  stock  in  un- 
treated soil.  Don't  transplant  infested  seed- 
lings into  treated  soil.  Don't  place  clean  soil 
in  untreated  containers.  (Sec.  3.) 

Either  use  pathogen-free  stock  from  a 
specialist  propagator,  or  treat  your  own 
selected  material  with  heat  or  chemicals  to 
insure  its  health  status.  (Sec.   13.) 

Use  top  cuttings  from  the  cleanest  plants 
you    have,   grown    on    supports   up    off   the 


f  Since  it  is  impossible  to  foresee  all  possible 
transgressions,  only  those  actually  encountered 
in  commercial  production  have  been  listed. 
Specialist  propagation  of  pathogen-free  plant- 
ing stock  is  not  considered  here. 


ground.  Don't  use  cuttings  taken  from 
plants  at  or  near  soil  level  unless  you  are 
certain  of  their  freedom  from  disease.  Don't 
use  root  divisions  unless  absolutely  neces- 
sary. (Sees.  3  and   13.) 

In  taking  cuttings,  break  them  off  when 
possible,  rather  than  pinching  or  cutting 
them.  If  knives  are  used,  soak  one  in  disin- 
fectant while  using  the  other,  alternating 
them  at  frequent  intervals.  In  mother  blocks, 
knives  should  be  treated  before  starting 
each  new  plant.  (Sees.  3  and  13.) 

Place  clean  planting  material  only  on 
treated  surfaces  of  benches,  flats,  baskets, 
etc.,  or  on  previously  unused  newspaper  or 
wrapping  paper;  never  place  it  on  the 
ground.  (Sees.  3  and   13.) 

Don't  dip  clean  planting  material  in 
water  unless  absolutely  necessary.  Never 
dip  clean  planting  material  into  a  tank  used 
previously  for  infested  stock.  Don't  use 
hormone  solutions  on  cuttings  of  uncertain 
health;  dust  hormone  powders  onto  the  cut 
ends  of  stems.  (Sec.  13.) 

Discard  seed  flats  with  any  diseased 
seedlings,  or  if  it  is  absolutely  necessary  to 
use  them,  transplant  only  from  spots  remote 
from  diseased  areas.  (Sec.  3.) 

Segregate  propagation  activities  and 
mother    blocks   from    crop    production,   and 


[22] 


Fig.  7.  A  20-gallon  crock  containing  a  solution  of  1  gal.  of  commercial  formaldehyde  to  18 
gal.  water.  Before  use,  tools  are  dipped  into  the  solution  for  a  few  minutes  and  the  excess 
drained  or  rinsed  off,  or  allowed  to  volatilize.  Fig.  8.  (Right)  The  nozzle  of  the  water  hose 
should  be  kept  off  the  ground  when  not  in  use. 


isolate  them  from  commercial  areas.  Main- 
tain them  as  separate  operations.  (Sec.  13.) 

Avoid  handling  treated  soil  unnecessarily. 
Treat  soil  directly  in  the  containers  when- 
ever possible.  Don't  nervously  dip  hands 
into  bench  soil  while  conversing  near  by. 
Don't  unnecessarily  feel  the  soil  for  moisture 
content,  or  knock  plants  out  of  pots,  unless 
the  hands  are  clean.  (Sees.  3,  8,  1 1,  and  12.) 

Avoid  splashing  infested  soil  particles 
into  treated  soil.  Don't  walk  over  treated 
flats  of  soil,  expose  treated  soil  to  blowing 
dust,  or  kick  dust  into  treated  soil.  (Sec.  3.) 

Treat  tools  with  disinfectant  before  using 
in  treated  soil  (fig.  7).  Use  clean  or  treated 
cloth  or  papers  to  cover  seed  flats.  (Sec.  3.) 

Place  flats  on  2  x  4  timbers  treated  with 
copper  naphthenate  or  on  polyethylene 
sheets,  for  hardening  plants  outdoors. 
Never  place  flats  of  plants  directly  on  soil. 
(Sees.  3  and  12.) 

Steam  benches  or  beds  after  each  crop 
is  removed,  even  if  it  was  grown  in  pots  or 
flats.  If  the  glasshouse  is  free  of  plants, 
chemical  fumigation  may  be  used.  This  is 
sanitation  insurance.  (Sees.  8  and  12.) 

Hang  the  hose  nozzle  on  a  hook  on  the 
side  of  the  bench  when  not  in  use  (fig.  8); 
never  drop  it  on  the  ground.  (Sec.  3.) 

Wash  the  hands  after  working  with  any 


soil  or  planting  stock  not  known  to  be  clean, 
before  handling  clean  materials.  (Sec.  3.) 

Use  ditch  or  surface  irrigation  with  slow 
stream  of  water  (no  lateral  flow)  on  plants 
used  for  propagation  material.  Don't  use 
overhead  sprinkling  on  mother  blocks.  (Sees. 
3  and  13.) 

Place  all  new  planting  material  of  un- 
certain health  in  a  special  isolation  ward 
until  you  know  it  is  healthy.  Never  place  it 
in  the  middle  of  or  near  clean  plantings. 
(Sees.  3  and   13.) 

Don't  underrate  the  danger  of  introduc- 
ing an  organism  into  your  nursery  or  fields 
because  it  "sounds"  like  one  you  already 
have.  Consider  organisms  as  different- 
even  though  they  are  called  by  the  same 
name— until  they  are  proved  otherwise. 
(Sec.  15.) 

Remember  that  many  organisms  which 
cause  seedling  diseases  also  attack  mature 
plants,  perhaps  years  later,  reducing  yield 
or  killing  them.  Don't  use  palliative  meas- 
ures (soil  drenches,  sphagnum  cover,  etc.) 
against  seedling  diseases.  (Sec.  3.) 

Use  clean  stock  in  treated  soil  and 
containers,  and  practice  sanitation  to 
keep  them  disease-free.  Don't  fight 
diseases,  eliminate  them. 


[23] 


The  sanitation  requirement  is 
not  unreasonable 

One  of  the  principal  objections  by 
growers  to  the  whole  program  is  that  an 
impossible  degree  of  hospital  cleanliness 
is  demanded.  This  is  not  the  case;  it 
would  be  more  accurate  to  say  that  or- 
dinary household  cleanliness  is  expected. 
The  housewife  fully  cooks  pork  to  avoid 
giving  her  family  trichinosis,  and  uses 
only  pasteurized  milk  to  protect  them 
from  undulant  fever;  the  nurseryman 
treats  soil  to  prevent  infecting  his  plants 
with  nematodes  and  fungi.  The  house- 
wife washes  and  peels  fruits  and  vege- 
tables to  prevent  disease;  the  nursery- 
man should  use  only  pathogen-free  stock 
for  the  same  reason.  In  washing  dishes 
she  is  reducing  the  danger  of  spreading 
colds  among  her  family;  in  disinfesting 
the  flats,  pots,  and  so  on  he  is  only  taking 
comparable  precautions  for  his  plants. 
As  she  usually  does  not  use  food  that  has 
dropped  on  the  floor,  so  he  should  not 
place  clean  flats  of  seedlings  on  infested 
ground.  It  is  not  as  fussy  to  avoid  walk- 
ing across  flats  of  treated  soil  as  it  is  to 
avoid  tracking  mud  into  a  clean  kitchen. 
It  is  accepted  practice  to  sneeze  or  cough 
into  a  handkerchief  to  prevent  spreading 
colds;  similarly  a  grower  will  avoid 
scattering  infested  soil  in  watering  and 
handling.  One  is  at  least  as  likely  to 
spread  disease  among  plants  by  scooping 
up  soil  from  infested  beds  or  ground  in 
the  hose  nozzle  as  to  spread  sickness  by 
using  another's  drinking  glass.  Growers 
should  use  treated  tools  to  prevent  dis- 
ease spread  in  the  nursery  for  the  same 
reason  that  we  use  only  our  own  tooth- 
brush and  towel.  One  does  not  visit 
friends  while  suffering  from  mumps,  nor 
should  one  introduce  new  plants  of  un- 
certain health  into  the  middle  of  a  large 
planting  of  painstakingly  acquired 
healthy  stock. 

As  modern  civilization  evolved,  these 
accepted  modem  health  precautions  were 
first  unknown  or  ignored,  then  scorned 
as  Fussy,  then  grudgingly  adopted,  and 


finally  accepted  as  standard  procedures. 
Even  today  primitive  people  ridicule 
many  of  them.  Nursery  practice  for  dis- 
ease control  is  similarly  developing.  At 
which  evolutionary  level  do  you  stand  in 
your  attitude  toward  these  inevitable  de- 
velopments? 

Retardant  organisms 

Although  much  more  information  is 
required  before  the  use  of  retardant 
organisms  is  commercially  feasible,  its 
potentialities  for  nurserymen  are  great. 
(Sec.  14.) 

Rather  than  leaving  to  chance  the  re- 
contamination  organism  and  the  time  of 
its  introduction,  it  may  be  possible  to 
introduce,  after  soil  treatment,  a  specific 
organism.  This  would  be  selected  for  its 
ability  to  inhibit  the  development  of  sub- 
sequently introduced  pathogens  by  pro- 
ducing an  antibiotic  in  place,  by  compet- 
ing for  available  nutrients,  or  by  parasit- 
izing the  pathogen.  This  controlled  colon- 
ization of  the  soil  may  perhaps  best  be  ac- 
complished by  using  a  selected  group  of 
organisms,  retardant  to  pathogens, 
including  some  which  develop  at  the 
various  soil  temperature  and  moisture 
ranges  to  be  encountered.  At  least  one  of 
the  organisms  would  always  then  be  ac- 
tive. Because  of  the  delicate  balance  be- 
tween the  inhibitory  action  on  pathogens 
of  the  antibiotic  produced,  and  its  toxic 
effect  on  other  beneficial  organisms  and 
on  the  crop  itself,  the  more  uniform, 
duplicable,  and  controlled  the  soil  condi- 
tions are,  the  better  the  chance  of  success. 
The  adoption  of  the  U.  C.  system  of  soil 
mixes  thus  brings  closer  the  possible 
commercial  application  of  this  method 
by  stabilizing  many  of  the  conditions.  A 
suspension  of  the  beneficial  organisms 
could  be  atomized  over  the  surface  of  the 
treated  soil  in  flats  prior  to  planting.  The 
use  of  antagonistic  soil  organisms  and 
the  adoption  of  some  uniform  soil  sys- 
tem may  enter  general  nursery  practice 
together,  reinforcing  and  modifying  each 
other  in  the  process.   I  Sec.  14.) 


[24] 


Rhizoctonia  damping-off  has  been  ex- 
perimentally controlled  in  a  U.  C.-type 
soil  mix  under  conditions  entirely  com- 
parable to  those  of  a  California  bedding- 
plant  nursery.  Species  of  Myrothecium, 
Penicillium,  Trichoderma,  Streptomyces, 
and  Plicaria  prevented  Rhizoctonia 
damping-off  of  pepper  seedlings,  even 
when  inoculated  at  the  same  time  as  the 
parasite  (figs.  121,  122,  and  123).  Under 
some  conditions  the  presence  of  these  re- 
tardant  organisms  was  slightly  injurious 
to  the  host  as  well  as  the  parasite.  (Sec. 
14.) 

Nitrifying  bacteria  might  be  inocu- 
lated into  the  soil  at  the  same  time  as  the 
antagonists,  perhaps  reducing  the  time 
before  nitrate  becomes  available  to  the 
plants.  (Sec.  14.) 

No  success  has  been  attained  from 
adding  these  organisms  to  raw  soil  under 
natural  conditions.  The  microbiological 
flora  of  a  given  natural  soil  is  well  bal- 
anced and  has  increased  to  the  capacity 
of  nutrients  and  space  under  the  given 
environmental  conditions.  In  a  word,  it 
is  biologically  buffered  against  any  or- 
ganisms that  are  subsequently  intro- 
duced. Some  shock  is  necessary  to  upset 
this  balance  before  effective  addition  of 
beneficial  organisms  is  possible.  This 
shock  may  be  achieved  by  soil  treatment, 
by  addition  of  a  nutrient  particularly 
favorable  to  the  organisms  added  but  not 
to  others,  and  perhaps  by  modifying  soil 
moisture  or  temperature.   (Sec.  14.) 

Mechanization  in  the  Nursery 

The  California  nursery  industry  has 
not  been  slow  to  capitalize  on  the  adapt- 
ability to  mechanization  of  the  whole 
U.C.  system  of  soil  mixes  and  handling. 
Every  nursery  has  special  features 
around  which  mechanization  must  be 
designed.  Certain  general  ideas  of  wide 
utility  have  been  developed,  however 
(fig.  126).  (Sec.  17.)  Among  these  are 
the  ones  described  in  the  following  para- 
graphs. 


Mixing,  filling,  and 
treating  operations 

A  skip-load  tractor  is  used  to  transport 
the  soil  ingredients  from  the  storage  piles 
to  a  large  concrete  mixer,  in  which  they 
and  a  measured  amount  of  water  and 
fertilizers  are  mixed.  The  mix  may  be 
dumped  directly  into  a  mechanical  flat 
filler  (figs.  9  and  127),  can  filler  (fig. 
10),  or  pot  filler  (fig.  135),  after  which 
the  containers  are  stacked  on  wooden 
pallets.  Alternately,  the  soil  may  be 
dumped  into  a  bulk  soil  treater  and  from 
this  into  treated  containers  as  above.  The 
untreated  containers  and  soil  are  loaded 
into  a  steam  cooker  (figs.  6  and  131)  or 
stacked  in  piles  for  chemical  treatment 
(fig.  107),  the  handling  being  done  by 
a  fork-lift  tractor.  After  treatment  the 
containers  of  soil  may  be  stored  for  a 
few  days.  The  flats  are  coming  to  be 
subdivided  by  insertion  of  smaller  re- 
movable containers,  each  holding  a  dozen 
plants.  (Sec.  17.) 

Planting  and  transplanting 
operations 

Flats  have  been  planted  by  machinery 
when  large  seeds  were  used,  but  there  are 
some  difficulties  with  tiny  seed  ( snap- 
dragon, petunia).  Attempts  to  increase 
the  size  of  small  seed  by  pelleting  have 
been  only  partially  successful  because  of 
reduced  germination.  Successful  equip- 
ment to  space-plant  tiny  seed  may  soon 
be  available.  At  present,  seed  is  sown  in 
flats  for  later  hand-transplanting;  indeed 
it  is  possible  that  this  laborious  process 
may  not  soon  be  abandoned,  because  it 
provides  the  opportunity  to  group  uni- 
form-sized plants  together.  To  balance 
this  point,  however,  there  is  the  delayed 
development  (10  to  20  days)  resulting 
from  the  transplanting  operation.  (Sees. 
16  and  17.) 

Plants  for  field  use  (celery,  peppers, 
eggplant,  tomato,  or  plants  grown  for 
flower-seed  companies  or  cut-flower 
growers)  have  been  grown  from  me- 
chanically   seeded    flats.    When    this    is 


[25] 


:■■.;.■ 


Fig.  9.  Equipment  for  filling  flats  with  soil.  Fork-lift  tractor  for  conveying  stacks  of  flats  on 
pallets  to  treating  equipment.  (Photo  courtesy  of  American  Plant  Growers,  Lomita,  California.) 
See  also  fig.  127. 


done,  the  seed  may  be  sown  by  a  vacuum 
plate  and  then  covered  with  tissue  paper 
and  sterile  sand.  The  flats  are  then 
watered,  stacked,  and  placed  in  a  germi- 
nation room  of  high  humidity  and  con- 
stant temperature.  Some  growers  cover 


them  with  polyethylene  sheets,  or  place 
individual  flats  in  bags  of  this  material. 
When  the  seedlings  are  just  emerging, 
the  flats  are  moved  into  the  glasshouse 
on  temporary  lines  of  steel  rolls.  Such 
handling  requires  absolute  freedom  from 


Fig.  10.  Automatic  filler  for  2-  or  5-gal.  cans  which  places  soil  in  containers  and  forms  a 
central  depression  in  which  the  liner  is  planted.  (Photo  courtesy  of  Oki  Nursery,  Perkins,  Cali- 
fornia.) See  also  fig.  135. 


damping-off,  and  must  be  confined  to 
large  lots  of  the  same  variety.  It  saves 
using  the  glasshouse  for  as  much  as  10 
days  during  germination,  and  provides 
more  uniform  moisture  conditions  with 
less  labor  than  can  be  maintained  in  the 
open  glasshouse.  Adaptations  of  these 
methods  have  been  made  for  planting 
in  cans  and  pots,  and  such  equipment  is 
presently  being  tested  by  growers.  (Sees. 
16  and  17.) 

Watering 

Watering    of    the    containers    in    the 
glasshouse   may   be    done   mechanically 


with  overhead  sprinklers  or  other  meth- 
ods. In  some  cases  fertilizers  have  also 
been  applied  in  this  way,  perhaps  fol- 
lowed by  a  water  rinse.  The  excellent 
drainage  and  aeration  of  the  U.  C.-type 
mix  greatly  facilitates  this  method  of 
watering,  as  there  is  slight  danger  of 
overwatering.  The  freedom  from  disease 
reduces  the  possibility  of  spread  of  or- 
ganisms during  this  operation.  Further- 
more, it  has  been  found  that  seedlings  de- 
velop faster  (5  to  11  days  for  celery)  in 
a  U.  C.-type  mix  than  in  standard  ma- 
nure formulas.  (Sees.  16  and  17.) 


THE  FUTURE 


A  unified  positive  system  for  the  pro- 
duction of  healthy  nursery  stock  is  out- 
lined in  this  manual.  Maximum  benefit 
will  be  obtained  from  the  adoption  of 
the  complete  integrated  program,  and  its 
potential  merits  cannot  adequately  be 
judged  from  the  results  of  using  one 
phase  of  it.  Some  parts  (for  example,  the 
soil  mixes)  will  prove  helpful  if  used 
alone.  Others  possibly  could  lead  to  ap- 
preciable losses  (for  example,  soil  treat- 
ment, followed  by  planting  with  infested 
stock).  In  order  to  achieve  maximum 
effectiveness  and  economic  benefit,  the 
grower  must  alter  his  procedures  to  mesh 
with  the  U.  C.  system.  In  this,  as  in  re- 
ligion, partial  conversion  is  likely  to  lead 
to  backsliding. 

The  shift  of  a  nursery  to  the  U.  C. 
system  is  not  as  difficult  as  it  may  sound. 
While  numerous  production  practices 
may  require  modification,  the  changes 
develop  logically  and  progressively. 
Change  in  the  attitude  of  the  grower  may 
be  more  troublesome,  often  presenting 
the  biggest  obstacle  to  success.  For  this 
reason,  the  easiest  and  most  rapid  adop- 
tion of  the  method  has  usually  been  by 
intelligent,  resourceful  growers  new  to 
the  California  nursery  business,  and 
without  preconceived  ideas.  Established 
growers  who  have  made  the  change,  how- 
ever, usually  agree  that  the  results  have 


been  worth  the  effort.  As  one  elderly 
nurseryman  phrased  it,  "It's  what  you 
learn  after  vou  think  von  know  it  all 
that  counts."  Minds,  like  parachutes, 
function  only  when  they  are  open! 

With  the  increasing  use  of  uniform 
soil  mixes,  soil  treatments,  and  healthy 
seed  or  planting  stock,  and  the  trend 
toward  mechanization  and  the  package 
marketing  of  plants,  any  plan  for  the 
future  must  take  them  into  account.  It 
is  probable  that  in  some  cases  a  cen- 
tralized service  for  providing  a  uniform 
treated  soil  may  develop,  as  it  has  in 
England.  The  soil  may  then  be  delivered 
into  bins  at  each  nursery  in  a  treated 
condition  ready  for  use.  Alternatively, 
the  bins  may  be  fitted  with  a  perforated 
pipe  grid,  and  the  soil  steamed  in  place, 
the  steam  being  supplied  by  the  grow- 
er's boilers  or  by  a  portable  steam- 
generating  service.  In  any  case,  nitrify- 
ing organisms  and  those  antagonistic  to 
pathogens  can  be  added. 

It  is  certain  that  changes  will  come 
rapidly.  Sometimes  these  may  be  neces- 
sary in  order  to  cope  with  disease  prob- 
lems, sometimes  because  of  competitive 
pressure  for  cheaper  production,  at  times 
for  other  reasons.  The  only  certainty  is 
that  the  trend  will  always  be  toward  the 
least  expensive  production  of  the  best 
possible  plants. 


[27] 


SECTION 


Today's  Nursery 
Problems 


Kenneth  F.  Baker 


The  California  nursery  industry 
Effect  of  economic  changes 


Mechanization  and  disease  control 


Causes  of  disease 


.he  INFORMATION  developed  in  this 
manual  concerns  the  complex  problems 
of  nurserymen  and  flower  producers  who 
grow  crops  in  prepared  soil  confined  in 
flats,  pots,  cans,  beds,  or  other  contain- 


ers. Several  of  the  features  of  California 
and  its  nursery  industry  have  such  an 
important  bearing,  directly  or  indirectly, 
on  disease  control  in  these  crops  that  they 
are  considered  here. 


THE  CALIFORNIA  NURSERY  INDUSTRY 


The  California  nursery  industry  is  an 
important  part  of  the  agricultural  econ- 
omy of  the  state  and  nation.  According 
to  the  1950  U.  S.  Census  of  Agriculture, 
the  state  leads  in  the  production  of  all 
nursery  stock.  The  wholesale  value  of  the 
crops  grown  in  1949  by  the  535  produc- 
ing nurseries  (out  of  2.500  in  the  state) 
that  filed  returns  was  $10,789,239,  or 
15.2  per  cent  of  the  national  total.'  This 
was  more  than  the  next  two  states  com- 
bined, California  also  led  in  the  produc- 
tion of  ornamental   nursers   slock   (15.0 

'That  this  figure  is  very  conservative  is 
shown  l.\  the  L954  farm  valuation  ($33,324,980) 
of  nursery  stock  in  \2  southern  California  coun- 
ties. This  was  tenth  among  66  farm  commodities 
for  the  area. 


per  cent  of  the  U.  S.  total)  ;  floricultural 
plants,  rooted  cuttings,  and  other  ma- 
terial for  growing  on  (17.8  per  cent)  ; 
bedding  and  vegetable  plants  ( 16.5  per 
cent)  ;  and  lining-out  stock  (10.9  per 
cent) . 

There  were  6,676  nurseries  and  other 
outlets  licensed  for  plant  sales  in  the  state 
in    1954-55. 

There  are  about  300  growers  of  bed- 
ding plants,  farm  plants,  and  cacti  in 
the  stale.  About  80  per  cent  of  the  bed- 
ding and  farm  slock  is  produced  in  and 
south  of  San  Luis  Obispo,  Kern,  and 
San  Bernardino  counties,  and  about  52 
per  cent  of  the  stale  total  is  grown  within 


5    miles    of    Garden  a,    in    Los    Angeles 


I  28  ] 


County.  It  is  estimated  that  2,500,000 
flats  of  bedding  and  vegetable  plants 
were  produced  in  the  state  in  the  year 
ending  July  31,  1953.  Of  this  number 
1,580,000  flats  were  sold  in  the  state.2 

It  was  estimated3  that  10,000,000  1- 
gallon  and  500,000  5-gallon  cans  were 
used  in  California  nurseries  in  1952. 

In  order  to  produce  such  quantities 
of  container-grown  nursery  stock  in  Cali- 
fornia an  enormous  volume  of  special 
soil  mixes  must  be  used.  The  bedding- 
plant  growers  use  an  estimated  39,000 
cubic  yards  annually,  and  the  can  nur- 
series an  additional  55,000  cubic  yards. 
It  is  estimated4  that  the  2,500  nursery 
growers  in  the  state  had  an  average  of 
one  acre  per  nursery  in  container  pro- 
duction. Using  a  conservative  150  cubic 
yards  of  soil  per  growing  acre  per  year, 
375,000  cubic  yards  would  be  required 
annually  for  container  growing. 

A  conservative  estimate  of  the  amount 
of  soil  mixes  used  for  container-grown 
stock  would  be  350,000  cubic  yards, 
with  the  possibility  that  it  may  reach 
500,000  cubic  yards.  This  represents  the 
top  soil  (1-foot  depth)  of  217  to  300 
acres  of  land.  This  does  not  include  flori- 
cultural  stock  grown  in  beds  and  benches, 
a  use  which  involves  additional  large 
amounts  of  soil. 


Diseases  are  important 

Because  much  of  the  propagative  ma- 
terial produced  in  California  is  distrib- 
uted over  the  United  States,  its  disease 
status  is  very  important  and  of  more 
than  local  interest.  Increasing  attention 
has,  therefore,  been  devoted  to  devising 

2  Figures  in  this  paragraph  are  based  on  data 
kindly  supplied  by  J.  L.  Mather,  Manager  of 
the  former  Bedding  Plant  Advisory  Board, 
Bureau  of  Markets,  California  State  Depart- 
ment of  Agriculture. 

3  Figures  prepared  by  E.  J.  Merz,  Executive 
Secretary,  California  Association  of  Nursery- 
men, for  an  O.P.A.  investigation  of  can  use. 

4  By  W.  F.  Hiltabrand,  Nursery  Service,  Cali- 
fornia State  Department  of  Agriculture. 


means     for      producing      pathogen-free 
plants. 

Wide  variety  of  crops 

The  nursery  industry  in  the  state  pro- 
duces a  wide  variety  of  ornamental 
crops,  including  bedding  plants,  propa- 
gating stock  (for  example,  poinsettia  and 
geranium  cuttings),  started  plants 
(roses,  azaleas,  palms),  foliage  plants, 
succulents  and  cacti,  trees  and  shrubs, 
and  herbaceous  ornamentals.  Pathogen- 
free  seedlings  (pepper  and  tomato,  for 
example)  are  also  grown  for  the  vege- 
table industry.  Thus  nurserymen  here 
grow  hundreds  of  kinds  of  plants  and  in 
all  sorts  of  combinations.  The  term 
"nursery"  as  used  in  California  includes 
many  more  kinds  of  crops  than  in  other 
areas  of  the  country,  where  it  generally 
refers  to  woody  stock,  largely  grown  in 
the  field. 

This  diversity  of  crops  makes  it  im- 
possible to  reduce  nursery  practice  to  a 
rule-of-thumb.  In  this  manual,  therefore, 
the  general  methods  for  disease  preven- 
tion are  outlined,  and  facts  are  given 
from  which  a  sound  program  can  be  de- 
veloped for  the  specific  crops,  layout, 
and  location  of  a  given  nursery. 

Semiarid  coastal  climate 

Nursery  production  is  concentrated  in 
the  counties  of  Los  Angeles  (6.9  per  cent 
of  the  national  total),  Alameda,  San 
Bernardino,  San  Diego,  Riverside,  Santa 
Clara,  Orange,  Merced,  Tulare,  Ventura, 
San  Joaquin,  and  Kern,  in  descending 
order.  Most  of  the  ornamental  crops  ex- 
cept roses  are  produced  in  the  cool 
coastal  zone. 

The  sunny,  generally  mild,  semiarid, 
coastal  climate  enables  the  nurseryman 
to  modify  his  environment  more  com- 
pletely than  is  possible  in  other  compa- 
rable growing  areas.  It  is  more  efficient 
to  control  soil  moisture  through  irriga- 
tion than  by  building  rain  shelters,  to 
shade  plants  from  the  sun  than  to  use 
supplemental   lights,    and   to   accept   re- 


[29] 


gional  temperatures  than  to  heat  against 
subzero  weather.  This  climate  also  ef- 
fectively limits  certain  plant  diseases. 
For  example,  azalea  flower  blight  causes 
severe  losses  in  southeastern  states  but 
is  important  in  southern  California  only 
where  a  "southeastern  climate"  is  created 
by  the  continuously  moist  conditions  un- 
der lath.  The  possibility  of  climatic  con- 
trol in  California  is  great,  and  the 
disease-control  problem  is  simplified  by 
this  fact. 

Salinity 

There  is,  however,  another  side  to  the 
picture.  Because  of  the  semiarid  climate, 
agriculture  in  California,  particularly 
the  southern  part,  continually  faces  a 
salinity  problem.  Most  other  growing 
areas  have  trouble  with  salinity  only 
from  accumulation  of  fertilizers  in  the 
soil  due  to  deficient  leaching.   Nursery 


plants  in  the  southwestern  states  may  be 
injured  by  naturally  occurring  salts  in 
the  irrigation  water  or  soil,  as  well  as  by 
too  much  manure  or  decomposed  leaf 
molds  and  composts  (Sec.  4).  Although 
the  nursery  industry  uses  irrigation  and 
special  soil  mixes  in  containers,  it  has 
constant  and  sometimes  severe  losses 
from  excess  soluble  salts.  One  of  the  ad- 
vantages of  the  methods  described  in  this 
manual  is  that  they  reduce  this  salinity 
problem. 

Year-round  demand 

The  climate  has  still  another  effect  on 
the  nursery  industry  here.  This  is  one 
of  the  few  regions  in  the  country  where 
gardening  and  the  demand  for  nursery 
stock  continue  through  much  of  the  year. 
Although  there  is  considerable  seasonal 
variation  (fig.  11),  the  market  is  pro- 
longed and  stable  as  compared,  for  ex- 


August 


Fig.  11.  Distribution  of  wholesale  sales  of  bedding  plants  in  California,  by  months,  1951-52. 
Nearly  75  per  cent  of  the  stock  is  sold  in  the  6  months  from  February  through  June,  and  in 
October.  (Figures  from  the  former  Bedding  Plant  Advisory  Board,  Bureau  of  Markets,  California 
State   Department  of  Agriculture.) 


I  30  1 


ample,  with  that  in  the  northeastern 
states  with  freezing  winter  temperatures. 
This  fact  has  made  it  profitable  to  mecha- 
nize operations  to  a  greater  degree  than 
in  most  other  areas. 

Smog  injury 

This  problem  is  most  acute  in  the  Los 
Angeles  area,  but  is  appearing  elsewhere 
as  industrialization  and  urbanization 
proceed.  Even  if  reduction  of  injury  to 
crops  eventually  is  possible,  it  will  be 
expensive,  directly  or  indirectly,  and  will 
be  an  added  economic  burden  on  the 
nurseryman.  The  effects  are  so  serious 
that  many  growers  are  moving  to  smog- 
free  rural  areas.  When  a  nursery  thus 
moves,  an  opportunity  is  afforded  to  de- 
velop the  new  unit  along  lines  of  a  mech- 
anized U.  C.  system. 

Pathogen-free  planting  stock 

There  is  a  general  trend  toward  the 
use  of  pathogen-free  propagating  mate- 
rial. This  is  evidenced  by  increasing  use 
of  mother  blocks,  of  certification  of  bud- 
wood,  plants,  and  seeds  by  various  state 
agencies,  and  by  the  appearance  of  spe- 
cialist propagators.  The  grower  response 


to  this  control  of  disease  at  source  is 
illustrated  by  the  situation  of  chrysanthe- 
mum cuttings.  In  1943  a  method  was 
developed  by  A.  W.  Dimock  for  selecting 
chrysanthemum  stock  plants  free  of  Ver- 
ticillium  wilt,  and  this  was  adopted  by 
an  Ohio  propagator.  In  1949  nearly  26V2 
million  cuttings  were  produced  in  Ohio 
(69.5  per  cent  of  the  U.  S.  total) ,  largely 
by  this  concern !  The  use  of  healthy  pro- 
pagative  stock  eliminates  one  of  the  im- 
portant sources  of  disease  organisms, 
and  is  fundamental  to  the  U.  C.  system. 

Unit  containers  for  marketing 

For  more  effective  retailing  of  plants. 
California  growers  developed  the  use  of 
unit  containers.  Bedding  plants  are  pro- 
duced in  thin  wood  veneer,  aluminum, 
or  molded-asphalt  packages  holding 
about  a  dozen  plants.  This  method  of 
growing  requires  a  uniform,  well-aerated 
soil,  and  freedom  from  disease  organ- 
isms. The  use  of  cans  for  woody  plants 
in  California  and  Florida  has  made  such 
stock  available  all  year  for  transplanting, 
in  contrast  to  the  usual  short  planting 
season  for  stock  dug  and  balled  in  burlap 
or  heeled  in. 


EFFECT  OF  ECONOMIC  CHANGES 


Recent  economic  changes  in  California 
are  making  it  increasingly  important  to 
reduce  the  cost  of  fighting  disease,  and 
to  avoid  the  occasional  heavy  losses  they 
may  cause.  These  changes  also  make  it 
more  important  to  find  ways  of  cutting 
labor  cost  and  saving  space,  and  make 
mechanization  more  necessary. 

Land  values,  tax  rates,  and  zoning 
restrictions  are  increasing 

Increasing  population  pressure  in  the 
state  is  bringing  about  real-estate  devel- 
opment, rising  land  values,  higher  taxes, 


and  zoning  restrictions.  From  1940  to 
1950  the  population  increased  by  slightly 
more  than  one  half,  and  one  third  of  the 
state's  dwelling  units  were  constructed. 
In  the  Los  Angeles  and  San  Francisco- 
Oakland  areas  the  population  increased 
37.6  and  41.6  per  cent  respectively,  dur- 
ing that  period.  Growers  are  finding  that 
these  developments  collectively  create 
one  of  the  wrorst  pressures  they  face  to- 
day, and  many  are  contemplating  mov- 
ing to  rural  areas. 

On  the  other  hand,  the  larger  popula- 
tion will  provide  an  expanding  local 
market    and    eventuallv    reduce    out-of- 


[31] 


state  shipment.  The  immediate  effect, 
however,  is  to  sharpen  the  interest  in 
techniques,  such  as  soil  treatment,  that 
will  reduce  cost  of  production. 

In  addition  to  escaping  the  population 
pressures  and  the  increasing  air  pollu- 
tion, a  move  may  be  advantageous  in 
other  ways.  It  provides  an  opportunity 
to  clean  up  a  nursery  and  make  a  fresh 
start  along  the  lines  described  in  this 
manual.  It  is  also  an  ideal  time  to  mecha- 
nize, for  this  almost  always  involves  re- 
design of  the  layout,  and  is  best  done 
on  a  new  site.  In  some  cases  the  increased 
value  of  the  city  land  pays  a  large  part 
of  the  c«st  of  establishing  a  new,  modern, 
mechanized  nursery.  The  mechanization 
in  turn  brings  reduced  labor  cost,  in- 
creased efficiency,  and  lower  production 
cost. 

The  new  rural  location  need  not  be 
on  the  best  level  land.  Indeed,  there  are 
some  advantages,  such  as  lower  initial 
cost  and  possible  utilization  of  gravity- 
flow  operation,  in  side-hill  locations 
(Sec.  17) .  Present  grower  experience  in- 
dicates, furthermore,  that  distance  from 
market  is  not  as  much  of  an  obstacle  as 
it  once  was.  Eventually  the  various  pres- 
sures may  partially  offset  each  other  and 
hasten  the  modernization  of  the  Califor- 
nia nursery  industry. 

Labor  costs  are  increasing 

Labor  costs  are  increasing  rapidly 
because  of  both  increased  pay  scale  and 
decreased  work  output.  This  has  led 
directly  to  a  growing  interest  in  labor- 
saving  methods  and  devices,  mechaniza- 
tion, and  reduction  of  erratic  crop  losses. 


Returns  are  decreasing 

There  are  many  indications  that  com- 
petition is  intensifying  in  the  nursery 
business,  and  that  the  financial  returns 
to  growers  are  decreasing.  There  are  two 
ways  nurserymen  can  meet  this  situation. 

1.  They  may  reduce  production  cost 
through  improved  culture,  mechaniza- 
tion, and  reduction  of  erratic  unneces- 
sary losses  from  diseases  and  similar 
factors. 

2.  They  may  reduce  competition  by 
growing  plants  that  other  nurserymen 
find  difficult  to  produce  profitably,  rather 
than  those  found  in  most  establishments. 
The  "difficult"  crop  is  usually  one  that 
requires  such  painstaking,  specialized, 
or  highly  skilled  techniques  for  success 
that  most  growers  are  unwilling  or  un- 
able to  produce  it  profitably.  This  may 
be  due  to  the  necessity  of  controlling 
some  serious  disease,  or  to  the  develop- 
ment and  exclusive  retention  of  a  supe- 
rior crop  variety.  Some  specialists,  for 
example,  grow  pathogen-free  propaga- 
tive  stock  of  chrysanthemums,  carna- 
tions, geraniums,  or  foliage  plants.  If 
the  propagator  produces  healthy  stock 
at  reasonable  cost,  growers  come  to  de- 
pend on  him  as  a  source  of  supply.  Other 
specialists  are  developing  F1  hybrid 
flower  and  vegetable  seed  that  may  be 
purchased  only  from  the  originator. 

However  accomplished,  such  speciali- 
zation leads  to  a  limited  natural  monop- 
oly and  reduced  competition.  The  more 
difficult  the  problem,  the  better  the  job 
is  done,  and  the  more  reasonable  the 
charges  for  it,  the  greater  the  chance  of 
thus  reducing  competition. 


MECHANIZATION  AND  DISEASE  CONTROL 


For  reasons  already  mentioned,  mech- 
anization is  a  present  and  future  fact  in 
the  nursery  industry.  This  in  turn  im- 
poses certain   demands,   most   of  which 


are  in  themselves  beneficial.  Scheduled 
production  at  low  cost  demands  dependa- 
ble results.  Just  as  assembly-line  manu- 
facturing requires  that  no  phase  of  the 


32 


process  break  down,  so  scheduled  mecha- 
nized production  of  plants  demands  that 
all  possible  chances  for  error  or  failure 
be  removed.  Uncontrolled  losses  from 
diseases,  such  as  damping-ofl  in  seed 
flats,  must  be  eliminated  or  reduced  to 
unimportance,  or  the  rest  of  the  growing 
procedure  may  be  stopped  for  lack  of 
material. 

Mechanization  may  lead  to  bigness, 
since  some  kinds  of  equipment  (for  ex- 
ample, flat-making  or  can-filling  ma- 
chines)   can   profitably   be   added   only 


when  the  volume  has  reached  a  certain 
level.  This  bigness  eventually  may  intro- 
duce other  problems,  as  it  becomes  im- 
possible for  the  man  who  built  the  suc- 
cessful enterprise  to  maintain  personal 
supervision.  The  daily  application  of  his 
knowledge,  experience,  and  foresight 
often  is  the  price  of  maintained  success, 
and  very  large  nurseries  may  exhibit 
slackness  or  inefficiency  for  this  reason. 
Increase  in  size  places  ever  greater  em- 
phasis on  assured  control  of  diseases, 
insects,  and  soil  problems. 


CAUSES  OF  DISEASE 


Many  explanations  are  offered  for 
failure  of  a  nursery  crop,  and  these  often 
confuse  rather  than  clarify.  Thus  it  is 
said  that  flats  of  seedlings  have  been  wa- 
tered too  much  or  too  little,  the  seedlings 
were  planted  too  deep  or  too  shallow, 
the  weather  was  too  hot,  the  plants  were 
too  soft  or  too  hard,  or  came  from  a 
poor  lot  of  seed,  or  were  grown  in  the 
wrong  soil  mix.  In  most  cases  investiga- 
tion has  revealed  that  pathogens  had 
caused  the  disease,  and  that  the  condi- 
tions blamed  had  merely  aggravated  the 
trouble. 

This  confusion  is  understandable  in 
view  of  the  complexity  of  some  of  the 
situations  encountered.  Perhaps  the  com- 
monest example  in  southern  California 
is  the  damping-off-salinity  complex.  A 
grower  who  uses  untreated  composted 
soil  that  is  high  in  both  soluble  salts  and 
damping-off  fungi  truly  faces  a  dilemma. 
If  he  keeps  the  soil  on  the  dry  side  to 
reduce  fungus  attack,  the  plants  will  be 
injured  by  the  increasing  concentration 


of  salts.  If  he  tries  to  avoid  salinity  in- 
jury by  leaching  out  the  salts,  or  by  keep- 
ing the  soil  wet  to  dilute  them,  favorable 
conditions  may  be  created  for  damping- 
off  fungi  (fig.  35).  Indeed,  this  situation 
sometimes  is  seen  in  a  single  flat  contain- 
ing poorly  leveled  soil.  The  water  runs 
to  the  low  parts  and  leaches  them,  and 
the  seedlings  there  grow  well  until  they 
damp-off.  In  the  high  parts  of  the  flat, 
the  seedlings  are  stunted  and  hardened 
by  salt  concentration,  but  have  little 
damping-off.  The  only  real  solution  to 
this  situation  is  to  keep  the  salinity  at  a 
low  level,  and  to  treat  the  soil  to  free 
it  of  pathogens. 

Growers  may  be  further  confused  be- 
cause many  factors  that  may  aggravate 
disease  once  it  appears,  will  not  initiate 
it.  It  is  one  purpose  of  this  manual  to 
explain  the  action  of  the  several  factors 
in  this  disease  complex  so  that  growers 
will  have  a  rational  basis  for  its  under- 
standing and  prevention. 


[33  1 


SECTION 


Damping-OfT  and 
Related  Diseases 


Kenneth  F.  Baker 


D 


Development  of  methods  for  disease  control 
Nursery  diseases  and  their  pathogens 
The  Rhizoctonia  story 

The  water  molds,  Pythium  and  Phytophthora 
Other  organisms  that  cause  nursery  diseases 
Control  of  nursery  diseases 


iseases  OF  California  nursery  crops 
range  from  seedling  damping-off  to  leaf 
spots  and  stem  cankers,  fire  blight  and 
crown  gall,  root  rots  and  flower  blights. 
Since  this  manual  is  concerned  with  dis- 
eases in  relation  to  nursery  soil  in  con- 
tainers, its  coverage  is  largely  restricted 
to  damping-off  and  related  diseases  of 
propagative  material.  These  pathogens 
necessarily  involve  soil,  live  in  it,  or  use 
it  as  a  base  for  attacking  crops. 


Excluded  are  diseases  specific  to  single 
crops,  those  in  which  the  causal  organ- 
isms are  only  incidentally  associated 
with  soil,  and  those  that  parasitize  so 
slowly  that  symptoms  are  largely  shown 
in  the  post-nursery  phase.  Included,  how- 
ever, are  some  of  the  most  insidious, 
omnivorous,  tenacious,  and  destructive 
pathogens,  causing  some  of  the  worst 
headaches  of  the  nurseryman. 


DEVELOPMENT  OF  METHODS  FOR  DISEASE  CONTROL 


The  parallel  development  of  disease 
control  in  plants  and  man  is  instructive. 

In  early  times  (and  in  backward  areas 
even  today)  lack  of  understanding  of 
disease  led  to  invocation  of  supernatural 
causes.  Treatment  involved  atonement 
to  angry  deities,  and  therefore  involved 
application  of  unpleasant  noxious  mix- 
tures to  man  and  plants.  Surgery,  per- 
formed only  when  imperative  and  with- 
out regard  to  sanitation,  was  usually 
fatal. 

During  the  mid-nineteenth  century, 
after  microorganisms  were  shown  to 
cause  disease,  antiseptics  came  into  use 
to  destroy  pathogens  on  or  in  plant  or 
man.  The  mortality  from  disease  and 
surgei  \   fell  sharply.  'Flic  comparable  use 


of  sprays  on  plants  developed  rapidly, 
and  still  provides  one  of  the  principal 
methods  of  plant  disease  control. 

Toward  the  end  of  the  nineteenth  cen- 
tury the  concept  was  gradually  accepted 
that  preventing  the  introduction  of  or- 
ganisms was  better  than  trying  to  destroy 
them  on  or  in  the  host  by  antiseptics. 
This  gave  rise  to  the  aseptic  surgery  of 
today,  with  emphasis  on  sterilization  of 
equipment  and  on  general  cleanliness. 
Preventing  the  introduction  of  pathogens 
is  also  the  thesis  of  this  publication  on 
nursery  diseases,  expressed  in  the  motto, 
"Don't  fight  'em,  eliminate  'em."  This 
emphasizes  using  soil  and  plant  mate- 
rials free  from  disease  organisms,  and 
using  cultural  techniques  which  will  keep 


[34] 


them  that  way.  This  is  the  central  core 
of  the  U.  C.  system. 

In  the  present  day,  antibiotics  have 
revolutionized  many  aspects  of  medicine 
and  surgery.  They  may  conveniently  be 
injected  into  animals,  but  this  has  not 
been  very  successful  with  plants.  They 
have  only  a  local  fungicidal  effect  when 
sprayed  on  plants.  Since  antibiotics 
rapidly  break  down  when  introduced 
into  soil,  it  appears  that  best  results  there 
may  come  from  the  production  of  them 
by  the  appropriate  organisms,  in  place. 


Difficulties  must  be  resolved  before  this 
method  of  disease  control  comes  into 
wide  use,  even  under  the  uniform  con- 
trolled conditions  of  the  nursery.  This, 
then,  provides  another  reason  for  uni- 
formity of  soil  mix,  soil  treatment,  and 
handling.  The  use  of  antagonistic  soil  or- 
ganisms and  the  adoption  of  some  uni- 
form soil  system  may  enter  general 
nursery  practice  simultaneously,  rein- 
forcing and  modifying  each  other  in  the 
process;  these  topics  are  discussed  fur- 
ther in  sections  5  through  7,  and  14. 


NURSERY  DISEASES  AND  THEIR  PATHOGENS 


Types  of  Diseases 

Damping-off  of  seedlings 

The  several  types  of  damping-off  may 
occur  separately  or  simultaneously  in  a 
seedbed. 

1.  Seed  may  decay  before  it  germinates 
(the  seed-decay  phase)  or  the  seedling 
may  rot  before  it  emerges  from  the  soil 
(preemergence  damping-off,  fig.  12). 
Losses  of  these  types  are  usually  blamed 
on  defective  seed  because  of  the  poor 
emergence,  but  are  often  caused  by  a 
number  of  different  fungi. 

2.  Seedlings  may  develop  a  stem  rot 
near  the  soil  surface  and  fall  over  (fig. 
13).  This  postemergence  phase,  the  most 
conspicuous  type  of  damping-off,  is 
caused  by  Rhizoctonia  or  by  the  water 
molds. 

3.  Some  seedlings  may  be  so  tough,  or 
the  environment  so  unfavorable,  that  the 
stems  may  only  be  girdled,  and  the  plants 
remain  alive  and  standing  (figs.  1  and 
14).  Although  this  wire-stem  or  sore- 
shin  type  is  less  striking  than  the  former, 
it  is  just  as  destructive  because  the  plant 
is  stunted  and  eventually  dies.  It  usually 
is  caused  by  Rhizoctonia. 

4.  Another  postemergence  variant, 
commonly  caused  by  water  molds,  is  that 
in  which  the  rootlets  rot  from  the  tips 


(fig.  15)  ;  the  fungus  usually  progresses 
up  to  the  stem,  and  the  plant  dies. 

Top   rot  of  seedlings 
or  cuttings 

Under  moist  conditions  Rhizoctonia, 
or  sometimes  Phytophthora,  may  spread 
from  leaf  to  leaf,  or  stem  to  stem,  through 
the  tops  of  seedlings  or  cuttings  (fig. 
16).  It  may  rot  the  tops  down  to  soil 
level,  and  frequently  the  crown  and  roots 
are  uninjured.  The  fungus  may  originate 
from  the  soil,  spreading  up  the  first 
plants,  but  remain  aerial  thereafter. 

Cutting  and  stem  rot 

Cuttings  may  rot  progressively  from 
the  cut  end  (fig.  17),  the  root  bases,  the 
wounds  of  disbudding  (rose)  or  remov- 
ing of  basal  leaves  (gardenia),  or  from 
dead  leaf  bracts  (geranium) .  A  variant  of 
this  is  the  rot  of  the  propagative  piece  of 
Dieffenbachia  cane  by  Pythium  or  by 
soft-rot  bacteria  carried  over  in  tiny 
lesions  from  the  mother  cane.  Similarly, 
the  organisms  causing  bacterial  stem  rot 
of  geranium,  bacterial  wilt  of  carnation, 
and  bacterial  blight  of  chrysanthemum 
may  be  transmitted  unnoticed  in  the 
vascular  system  of  the  cutting  and  cause 
its  eventual  decay. 


[35] 


Root  rot  of  mature  plants 

The  roots  of  nursery  plants  in  pots, 
flats,  or  gallon  cans  may  rot  (much  as 
in  fig.  15)  and  cause  the  death  of  the 
plant.  The  tops  may  die  slowly,  with  yel- 
lowing and  dropping  of  leaves  beginning 
at  the  base,  or  the  leaves  may  rapidly 
wilt  and  die,  remaining  attached  to  the 
plant.  The  rate  of  symptom  expression 
in  the  tops  is  dependent  on  the  rapidity 
of  root  decay.  The  water  molds  usually 
cause  this  disease,  and  the  soil  moisture 
is  important  in  determining  the  rate  of 
root  decay. 

Time  and  Severity  of  Losses 

The  loss  to  the  nurseryman  may  be 
immediate,  as  in  damping-off  or  top  rot 
of  seedlings.  It  may,  however,  be  delayed 
and  cause  slow  loss  of  infected  plants, 
as  in  root  rot  caused  by  water  molds 
when  plants  are  grown  under  relatively 
dry  conditions.  It  may  cause  the  death 
of  plants  in  5-gallon  cans  several  years 
after  propagation  and  infection,  as  is 
often  the  case  of  Choisya  infected  with 
water  molds. 

Loss  may  even  be  so  delayed  that  it 
will  occur  after  the  plant  has  been  sold 
and  planted  in  a  home  yard.  Nurserymen 
often  do  not  learn  of  such  a  loss,  or  may 
feel  that  it  is  beyond  their  responsibility. 
Since  the  soil  becomes  infested  from  such 
a  plant,  other  clean  replacements  are 
likely  to  become  diseased  and  likewise 
die.  Although  a  very  few  nurserymen 
may  take  the  view  that  these  repeated 
losses  will  promote  plant  sales,  the  gross 
effect  undoubtedly  will  be  harmful  to  the 
industry.  The  buyer  may  become  a  dis- 
couraged gardener  and  poor  customer, 
may  change  nurseries  in  order  to  find 
better  stock,  or  may  learn  the  facts  and 
become  actively  antagonistic.  It  is  cer- 
tain that  the  sale  of  vigorous  pathogen- 
free  plants  is  one  of  the  best  ways  to 
build  a  good  reputation  and  promote 
sales,  both  for  a  single  nursery  and  the 
industry.  Thus  the  direct  or  immediate 


[36] 


losses  and  the  indirect  or  delayed  ones 
are  of  equally  great  importance  to  nur- 
serymen. 

Losses  from  these  types  of  disease  in 
nurseries  may  be  very  great.  Frequently 
50  per  cent  or  more  of  some  types  of 
plants  (Choisya,  succulents,  cacti,  Cali- 
fornia "natives")  die  before  sale.  When 
it  is  considered  that  the  margin  of  profit 
in  the  nursery  business  is  fairly  small 
and  becoming  smaller,  a  severe  disease 
loss  may  be  disastrous.  An  average  loss 
of  1  to  10  per  cent  from  diseases  would 
be  a  conservative  estimate.  This  must  be 
a  severe  drain  on  the  slender  margin  of 
profit  in  many  nurseries,  and  probably 
causes  many  of  them  to  operate  at  a  loss, 
at  least  on  some  items. 

Relative   Importance 
of  Pathogens 

In  California,  as  apparently  in  much 
of  the  world,  the  principal  cause  of  the 
diseases  mentioned  is  a  fungus  (Rhizoc- 
tonia  solani)  commonly,  though  not  af- 
fectionately, called  "rhizoc."  In  the  past 
10  years  this  fungus  has  become  increas- 
ingly important  as  a  pathogen  of  flower, 
nursery,  vegetable,  and  field  crops  in 
southern  California.  While  the  reason  for 
this  increase  is  not  known,  the  pathogen 
is  rapidly  becoming  the  most  important 
single  fungus  causing  crop  disease  in 
that  area. 

Of  less  general  importance,  the  water 
molds  (Pythium  and  Phytophthora  spp.) 
may  be  locally  damaging. 

Other  fungi  cause  infrequent  losses  to 
seedlings  and  cuttings,  but  are  of  minor 
importance. 

It  is  the  purpose  here  to  explain  and 
illustrate  the  nature  of  these  various 
fungi,  and  how  they  survive  and  spread, 
so  that  a  grower  may  better  understand 
what  is  happening,  and  better  plan  his 
preventive  program.  Rhizoctonia  is  dis- 
cussed in  detail  as  the  principal  example, 
and  discussions  of  less  important  organ- 
isms refer  to  it. 


[37] 


THE  RHIZOCTONIA  STORY 


Recognition  of  Rhizoctonia 
Diseases 

It  is  reasonably  certain  that  rhizoc  is 
the  cause  of  the  disease  when  (1)  the 
decay  originates  near  the  soil  surface 
(fig.  13) ,  rather  than  at  root  tips  as  with 
the  water  molds  (fig.  15)  ;  (2)  coarse 
brown  fungus  mycelium  is  seen,  with  the 
aid  of  a  good  10  to  15x  hand  lens,  on 
the  decayed  parts  (figs.  17,  18,  and  27)  ; 
(3)  there  are  soil  particles  clinging  to 
the  tough  fungus  strands  after  the  seed- 
ling is  shaken  to  remove  the  soil  mass 
from  the  roots  (fig.  18) .  Few  other  fungi 
{Helminthosporium  cactivorum  on  cac- 
tus is  one  of  the  exceptions)  have  been 
found  to  simulate  these  features  of 
Rhizoctonia;  the  fungi  can  of  course 
easily  be  differentiated  by  growing  them 
on  culture  media. 

Cuttings  may  rot  progressively  up- 
ward when  infection  has  occurred  below 
soil  level  (fig.  27).  In  humid  propagat- 
ing cases  Rhizoctonia  often  spreads  as  a 
coarse  web  through  the  tops  of  some 
plants  (for  example,  Araucaria  cuttings, 
azalea  cuttings  and  grafts) ,  matting  them 
together  (fig.  16). 

The  Rhizoctonia  Fungus 

Rhizoctonia  solani  is  a  simple  plant 
consisting  of  brown  threadlike  branch- 
ing mycelium.  The  presence  of  this 
characteristically  branching  mycelium 
(fig.  19,  top)  on,  and  particularly  in, 
diseased  tissue  affords  a  rapid  labora- 
tory microscopic  diagnosis  for  the 
fungus.  In  the  soil  these  filaments  grow 
between  the  particles  and  in  bits  of  or- 
ganic matter  (highly  magnified  in  fig. 
20).  Sometimes  these  strands  fuse  to- 
gether and  form  visible  clumps  (sclcro- 
lia.  fig.  19.  bottom)  that  are  long-lived 
and  survive  drying.  Thus,  infested  soil 
and  flats  ma\  be  stored  dry  for  6  months 
01    more    without     killing    the    parasite. 


When  the  fungus  grows  into  contact  with 
a  seedling  (figs.  12  and  13),  its  my- 
celium grows  over  the  surface  and  pene- 
trates the  tissues  (fig.  28),  digesting 
them  for  its  own  nutrition,  and  thus  pro- 
ducing the  disease. 

The  fungus  is  a  relatively  unspecial- 
ized  parasite,  able  to  attack  many  kinds 
of  plants,  although  there  are  important 
differences  in  this  ability  between  strains 
of  the  fungus  (Sec.  15). 

Spread   of  the   Fungus,  and 
Preventive  Measures 

Under  conditions  of  plant  propaga- 
tion, Rhizoctonia  produces  spores  ex- 
tremely rarely  or  not  at  all.  For  all 
practical  purposes  there  is  no  air-borne 
stage,  and  spread  occurs  by  mechanical 
transfer  of  mycelium  and  sclerotia  in  in- 
fested soil  particles  and  infected  plant 
tissue.  These  bits  of  mycelium  resume 
growth  in  the  new  location.  This  is  of 
great  significance  in  control  procedures. 

Rain  or  watering 

During  rain,  overhead  irrigation,  or 
watering  of  flats  and  beds  in  which  the 
fungus  occurs,  bits  of  soil  containing  its 
strands  commonly  are  spattered  to  near- 
by uninfested  plantings  (fig.  21). 

Dipping  cuttings 

Similarly,  spread  may  occur  from  dip- 
ping cuttings  in  water  or  in  hormone 
solutions.  Since  water  is  an  efficient  car- 
rier of  many  kinds  of  pathogens,  it 
should  not  be  used  as  a  dip  in  cutting 
treatments;  any  materials  should  be 
either  dusted  or  sprayed  on  instead. 

Soil  in  watering  hose 

The  soil  under  benches  in  greenhouses 
often  is  infested  with  Rhizoctonia.  When 
the  hose  is  dropped  on  the  ground  after 
use,  bits  of  infested  soil  may  get  into  the 


[38  1 


[39] 


open  end  (fig.  22)  and  be  washed  into 
a  clean  planting  when  next  used.  This 
is  the  principal  reason  why  heavy  disease 
losses  frequently  occur  in  beds  at  points 
near  the  faucet.  Nozzles  should  be  hooked 
on  the  side  of  benches  when  not  in  use 
to  keep  them  up  off  the  ground  (fig. 
8). 

Infested  containers 

Rhizoctonia  very  commonly  lives  over 
between  crops  in  bits  of  soil  on  the  wood 
and  in  the  corner  joints  of  flats  (fig.  23) . 
When  clean  or  treated  soil  is  placed  in 
such  a  flat,  and  irrigated  after  planting, 
the  fungus  resumes  growth  and  causes 
damping-off  in  the  corners  (fig.  23)  or 
along  the  sides.  It  is  carried  over  in 
benches  and  cold  frames  and  on  pots  in 
the  same  manner.  Flats  and  benches 
should  be  treated  with  steam  or  chemi- 
cals (Sec.  12)  after  each  use  to  prevent 
carryover  of  pathogens. 

Infested  tools  and  equipment 

Exposed  surfaces  and  cracks  in  tools 
and  equipment  such  as  shovels,  trowels, 
dibbles,  replanting  tools,  and  wheelbar- 
rows also  afford  a  place  for  survival  and 
spread  of  the  fungus  (fig.  24).  After 
use  in  infested  soil,  tools  should  be 
dipped  for  a  few  minutes  in  a  crock  con- 
taining 1  gallon  of  commercial  formal- 
dehyde to  18  gallons  of  water  (fig.  7)  ; 
they  may  be  rinsed  in  water  and  used 
without  delay. 

Grower's  hands  and  feet 

The  fingers  of  the  grower  may  also 
carry  bits  of  soil  and  the  fungus  from 
flat  to  flat  while  testing  for  moisture  or 
knocking  plants  out  of  pots  for  root  ex- 
amination. A  green  thumb  may  actually 
be  the  black  hand  for  seedlings!  This 
hazard  is  minimized  by  having  only 
treated  soil  in  clean  flats  in  the  green- 
house range. 

Equally  dangerous  and  unfortunately 
common  is  the  practice  of  walking  on 
the  edges  of  flats  in  ground  beds  while 


[40] 


watering,  for  infested  soil  particles  often 
drop  from  the  shoes  into  clean  flats. 

Placing  containers  on  ground 

It  is  poor  practice  to  place  clean  flats 
or  cans  on  ground  beds  at  all;  infested 
soil  may  be  kicked  or  splashed  into  flats, 
or  the  roots  grow  through  the  bottom  and 
become  infected,  the  fungus  then  spread- 
ing to  the  plant  above.  Outdoor  beds 
where  flats  are  to  be  placed  may  be 
drenched  with  formaldehyde  (1  pint  of 
commercial  formaldehyde  to  61/4  gal. 
water)  at  %  gallon  per  square  foot  of 
surface.  They  must  be  kept  moist  and 
aerated  for  10  to  14  days  before  use 
(Sec.  11).  Two  by  four  timbers  treated 
with  copper  naphthenate  (Sec.  12) 
placed  flat  on  the  ground  will  elevate  the 
flats  sufficiently  to  provide  inexpensive 
and  fairly  effective  protection.  If  flats 
are  placed  on  polyethylene  sheets  laid 
on  the  ground,  protection  is  also  af- 
forded. Some  nurseries  pave  the  area 
with  asphalt. 

When  treated  flats  are  stacked  on  un- 
treated ground  the  bottom  one  should 
be  discarded;  at  the  minimum  it  should 
never  be  stacked  among  other  clean  flats. 
A  good  practice  is  to  place  stacks  of 
treated  flats  on  clean  wooden  pallets  to 
keep  them  off  the  ground. 

Unsterilized  covers 

The  use  of  old  unsterilized  canvas  or 
sacking  over  seed  flats  is  a  common 
source  of  infestation  of  clean  soil  (fig. 
25).  Unsterilized  lath  frames  placed  over 
flats  may  also  be  dangerous.  All  such 
materials  should  be  steamed  or  treated 
chemically  before  re-use. 

Infected   plants  or  seeds 

Rhizoctonia  may  also  be  carried  to 
clean  soil  by  infected  (but  healthy  ap- 
pearing) plants  or  seed.  With  some  crops 
(tomato,  eggplant,  pepper)  the  fruit  in 
contact  with  the  soil  may  be  slightly  de- 
cayed by  the  fungus,  which  then  develops 
on   and  in  some  of  the  enclosed  seeds 


27 


CLEAN  SOIL 


28 


[41] 


I  fig.  26).  Zinnia  seedheads  piled  to  dry 
on  canvas  on  the  ground  may  similarly 
be  invaded  and  the  seed  infected.  Such 
transmission  of  Rhizoctonia  fortunately 
is  not  known  for  most  kinds  of  seeds. 

It  is  usual  practice  to  salvage  seedlings 
or  cuttings  from  the  margins  of  an  area 
of  damping-off  in  seed  pan  or  propagat- 
ing bench,  in  the  mistaken  notion  that 
only  decayed  plants  are  infected.  Ac- 
tually infection,  under  some  conditions, 
may  occur  a  few  days  before  symptoms 
appear.  To  transplant  such  stock  to  clean 
soil  is  to  transfer  the  Rhizoctonia  fungus 
(fig.  27).  A  wise  precaution  is  to  cover 
the  diseased  areas  with  inverted  tin  cans 
before  beginning  to  transplant,  making 
sure  that  the  can  extends  well  beyond 
the  margins. 

Root  divisions  or  basal  cuttings  of 
plants  such  as  chrysanthemums  grown 
in  infested  soil  commonly  carry  the 
fungus  to  the  new  planting  or  propaga- 
tion bed  (fig.  27).  This  carryover  can 
largely  be  avoided  by  using  only  cuttings 
from  tips  of  stems  a  foot  or  more  above 
the  soil,  since  the  fungus  is  not  carried 
to  that  height  by  splashing  water. 

The  parasite  invades  the  stems  of  the 
seedling  or  cutting,  and  once  inside 
the  tissues  is  very  well  protected  from 
any  fungicide  (fig.  28).  Because  of  this, 
it  is  not  possible  to  cure  a  diseased 
seedling  by  fungicidal  application.  Em- 
phasis must  be  on  prevention  of  infec- 
tion. It  is  possible,  however,  to  prevent 
spread  of  Rhizoctonia  from  a  small  in- 
fection in  a  flat  by  "spot  treatment"  of 
the  diseased  area  (Sec.  11). 

Conditions  Affecting 
Disease  Severity 

The  severity  of  attack  by  Rhizoctonia 
is  conditioned  by  the  susceptibility  of 
the  host,  the  inoculum  potential,  and  a 
number  of  environmental  conditions. 
Some  examples  of  these  factors  follow. 
In  general,  those  conditions  unfavorable 
to  the  plants  without  being  loo  detrimen- 


tal to  the  fungus  will  give  severe  disease 
losses.  The  more  unfavorable  the  condi- 
tions are  to  the  plant,  without  drastically 
reducing  growth  of  the  fungus,  the 
worse  the  disease  will  be. 

Susceptibility  of  the  host 

Peppers  consistently  suffer  heavier 
losses  from  Rhizoctonia  damping-off 
than  do  tomatoes,  and  pansies  or  stocks 
are  more  sensitive  than  calendula.  The 
sensitivity  of  the  plant  to  the  environ- 
mental conditions  given  below  must  also 
be  considered. 

Inoculum  potential 

The  quantity  of  Rhizoctonia  in  the  soil 
determines  the  potential  severity  of  the 
disease  and,  to  some  extent,  the  effective- 
ness of  control  procedures.  For  example, 
if  the  fungus  is  present  in  sufficient 
quantity  in  soil,  it  is  almost  impossible 
to  control  damping-off  by  chemical 
treatment  of  the  seed  or  by  treatment  of 
the  soil  by  the  dilute-formaldehyde 
method  (sees.  11  and  15).  The  environ- 
mental conditions  determine  whether  the 
potential  severity  is  attained. 

Soil  salinity 

Soil  salinity  (Sec.  4)  causes,  at  dif- 
ferent concentrations,  suppression  of 
germination,  stunting  of  plants,  killing 
of  the  margins  or  entire  blades  of  leaves, 
or  death  of  seedlings.  Experiments  at 
Pennsylvania  State  University  showed 
that  sublethal  salinity  increases  severity 
of  Rhizoctonia  damping-off.  This  may  ex- 
plain the  increasing  importance  of  this 
fungus  in  southern  California  in  the  dec- 
ade after  1944,  when  deficient  rainfall 
greatly  increased  the  salinity  problem. 

Nitrogen  and  carbohydrate 
status  of  plant 

The  higher  the  relative  level  of  soil 
nitrogen,  the  softer  the  plant  growth  will 
be.  Above  a  certain  level  this  increases 
susceptibility  to  Rhizoctonia  damping- 
off.  Favorable  light  for  the  plant  enables 


[42] 


it  to  produce  sufficient  carbohydrates  for 
thickened  cell  walls  and  sturdy  growth. 
To  a  considerable  extent,  nitrogen  sup- 
ply and  carbohydrate  level  (sunlight) 
offset  each  other.  A  plant  grown  at  a 
barely  adequate  nitrogen  level  in  sub- 
dued light  would  be  nitrogen-deficient 
in  bright  light,  and  one  with  adequate 
nitrogen  in  full  sun  would  have  too  much 
in  reduced  light.  Thus,  light  must  be 
considered  in  determining  an  adequate 
nitrogen  level.  Generally,  high-nitrogen 
or  low-carbohydrate  seedlings  are  sus- 
ceptible, whereas  the  hard  plants  of  low 
nitrogen  or  high  carbohydrate  are  more 
resistant. 

Watering 

Application  of  water  affects  plant  suc- 
culence and  susceptibility,  in  part 
through  affecting  nitrogen  intake.  At- 
tack by  water  molds  can  be  reduced  in 
severity  by  maintaining  the  soil  as  dry 
as  will  permit  plant  growth,  but  this  will 
not  inhibit  Rhizoctonia.  Rhizoctonia  con- 
trol through  reduced  watering  probably 
is  mainly  operative  on  the  plant  rather 
than  on  the  fungus. 

Soil  temperature 

The  growth  of  both  plant  and  fungus 
is  affected  by  soil  temperature,  but  the 
effect  is  often  unequal  in  degree  and 
range.  Thus,  a  strain  of  Rhizoctonia  may 
severely  injure  peas  (a  low-temperature 
crop)  at  high,  but  do  little  or  no  damage 
at  low  soil  temperatures,  and  attack 
beans  (a  high-temperature  crop)  at  low 
but  only  slightly  at  high  soil  tempera- 
tures. 

Depth  of  planting 

Deep  planting  of  seed  delays  emer- 
gence and  keeps  the  seedling  in  a  sus- 
ceptible state  (devoid  of  light  and  there- 
fore low  in  carbohydrate)  for  a  long 
period  of  time.  This  naturally  favors 
incidence  of  damping-off. 


Reduced  vitality  of  seed  . . . 

causes  delayed  emergence  of  the  seed- 
ling, with  much  the  same  effect  as  deep 
planting.  Old  weak  seed  may  have  more 
trouble  from  damping-off  than  new  seed 
of  high  rapid  germination. 

Rhizoctonia  Infections  on 
Mature  Plants 

Mature  plants  as  well  as  seedlings  are 
affected.  It  is  a  mistake  to  regard  damp- 
ing-off and  cutting-decay  fungi  as  limited 
to  juvenile  plants,  although  their  damage 
may  be  greatest  there.  Rhizoctonia 
causes,  in  addition  to  seedling  damping- 
off,  serious  losses  from  wire-stem  gir- 
dling of  mature  stocks,  peppers,  cabbage, 
and  other  plants  in  the  field,  as  well  as 
from  stem  rot  of  mature  carnations, 
pansies,  and  petunias.  It  has  also  caused 
serious  rot  of  the  deep  roots  of  nursery 
roses,  asters,  and  camellias.  The  water 
molds  may  even  assume  their  greatest 
importance  in  the  postnursery  growth. 
Transplanting  apparently  healthy,  but  in- 
fected, seedlings  to  the  field  or  bed  does 
not  end  the  matter,  for  they  frequently 
die  later  and  give  an  irregular  stand 
(fig.  29),  as  well  as  infest  the  soil  with 
the  fungus. 

Temporary  suppression 
is  not  control 

Some  measures  aim  at  suppression  of 
the  fungus  in  seedbed  or  propagation 
frame  by  the  use  of: 

1.  A    sand    or    sphagnum    moss    surface 

layer; 

2.  Reducing  watering; 

3.  Increased  aeration  of  seedlings; 

4.  Reduced  use  of  nitrogenous  fertilizers: 

5.  Increased  light; 

6.  Fungistatic    drenches     (for    example. 

PCNB),  or  even  fungicides  having 
poor  soil  penetration  (Arasan,  cap- 
tan). 


[43] 


These  palliative  treatments  may  be 
quite  effective  when  skillfully  applied, 
and  therein  lies  the  danger. 

The  fungus  may  be  suppressed  under 
the  controlled  conditions  of  the  seedbed 
or  flat,  only  to  appear  again  when  the 
plant  is  in  the  pot,  the  5-gallon  can,  or 
in  the  largely  uncontrollable  environ- 
ment of  the  commercial  or  home  plant- 
ing. Under  these  conditions  the  suppres- 
sive measures  may  be  ineffective  or  un- 
economical, and  it  finally  becomes 
evident  that  the  loss  has  been  merely 
postponed  until  the  investment  is  greater. 
(see  also  "The  Water  Molds,"  below). 

Strains  of  Rhizoctonia 

Rhizoctonia  strains  differ  in  response 
to  various  environments  and  hosts.  As 
pointed  out  in  Section  15,  it  is  unsafe  to 
assume  that  all  strains  of  Rhizoctonia 
solani,  which  are  fairly  common  and 
widespread,  are  alike  and  that  the  dis- 


tribution of  diseased  stock  is  therefore 
unimportant.  In  comparison  with  highly 
specific  soil  organisms,  such  as  the  wilt 
fusaria,  which  attack  only  a  single  species 
of  plant,  this  fungus  is  unspecialized ; 
but  there  are  differences  in  pathogenicity 
among  strains  of  Rhizoctonia  sufficient 
to  be  of  great  economic  importance. 
There  are  saprophytic  forms  of  R.  solani 
in  most  field  soils,  but  this  is  no  excuse 
for  bringing  in  virulent  pathogens  on 
the  stock  to  be  planted. 

In  addition  it  must  be  considered  that 
diseased  stock,  because  it  is  already  in- 
fected, will  suffer  more  rapid  and  severe 
injury  than  that  infected  in  the  field. 
Planting  diseased  stock  also  serves  to 
increase  the  quantity  of  pathogen  present 
and  to  distribute  it  more  uniformly 
through  a  field. 

For  all  of  these  reasons  it  is  important 
that  the  stock  produced  not  be  merely 
disease-free  (that  is,  healthy  appearing), 
but  that  it  be  pathogen-free  as  well. 


THE  WATER  MOLDS,  PYTHIUM  AND  PHYTOPHTHORA 


Types  of  disease 

Damping-off  caused  by  Pythium  or 
Phytophthora  usually  starts  at  tips  of 
main  or  lateral  roots  but  may  rapidly 
involve  all  parts  below  ground  (fig.  30) 
and  thus  cause  the  seedlings  to  fall  over. 
These  organisms  also  commonly  cause 
decay  of  seeds  or  seedlings  before  they 
emerge  from  the  soil  (as  in  fig.  12). 

The  fungi 

The  fungus  mycelium  on  the  roots  is 
fine,  colorless,  and  difficult  to  see  with  a 
hand  lens;  it  is  so  delicate  that  it  does 
not  hold  soil  particles,  as  does  that  of 
Rhizoctonia.  The  mycelium  grows  be- 
tween soil  particles  and  in  organic  mat- 
ter l  as  in  fig.  20)  through  the  top  several 
inches  of  soil;  it  is  therefore  in  a  posi- 


tion to  invade  root  tips.  These  fungi 
generally  are  damaging  to  plants  only 
when  the  soil  is  very  wet,  hence  the  name 
"water  molds."  Rhizoctonia,  on  the  other 
hand,  develops  best  under  conditions  of 
moderate  soil  moisture. 

Under  favorable  conditions  the  water 
molds  produce  in  soil  and  on  its  surface 
microscopic  saclike  structures  (zoos- 
porangia),from  which  emerge  numerous 
swimming  spores  (fig.  31).  These  swim 
about  in  water  for  a  time  before  develop- 
ing into  mycelium,  which  penetrates  a 
seedling  root. 

In  diseased  plants  these  fungi  com- 
monly develop  thick-walled  oospores 
(fig.  32),  which  are  long-lived  and  very 
resistant  to  drying.  These  spores  are  use- 
ful in  rapid  laboratory  microscopic  ex- 


[44] 


amination  of  suspected  roots  for  this 
group  of  fungi,  which  frequently  are 
difficult  to  isolate  in  culture. 

Spread 

The  water  molds  are  spread  in  the 
various  ways  just  described  for  Rhizoc- 
tonia,  and  in  addition  the  swimming 
spores  may  be  scattered  in  splashing 
drops,  irrigation  water,  and  so  on.  These 
motile  spores  may  develop  in  standing 
water  in  small  reservoirs  and  irrigation 
canals,  and  be  spread  by  using  such  in- 
fested water.  They  are  not,  however, 
found  in  city  water  supplies  in  Cali- 
fornia. 

The  oospores  are  released  to  the  soil 
by  decay  of  the  plant,  and  may  function 
there  in  the  same  way  as  sclerotia  of 
Rhizoctonia.  Therefore,  the  water  molds, 
despite  the  sensitivity  of  the  mycelium 
and  motile  spores  to  drying,  may  survive 
in  dry  soil  for  several  months  and  be 
spread  in  dry  soil  or  on  tools  or  flats. 

The  mycelium,  motile  spores,  or  the 
resting  spores  of  water  molds  are  not 
carried  by  air  currents.  The  water  molds 
may  be  introduced  to  clean  soil  with  in- 
fected seedlings  and  cuttings.  Phytoph- 
thora  also  is  known  to  be  carried  in  the 
seed  of  some  types  of  plants. 

Effects  on  mature  plants 

The  water  molds  are  important  in 
postnursery  phases  of  growth  as  well  as 
in  the  seedbed.  There  is  increasing  evi- 
dence that  water  molds,  as  well  as  some 
other  organisms,  may  retard  root  de- 
velopment (and  thus  plant  growth) 
without  invading  tissues.  Because  of  this 
effect,  as  well  as  actual  root  decay,  many 
of  the  water  molds  are  dangerous  both 
in  the  nursery  and  postnursery  phases. 
Phytophthora  cinnamomi  may  cause  de- 
cay of  tiny  heather  cuttings  and  the 
death  of  large  plants  in  cut-flower  fields; 
it  may  cause  loss  of  avocado  rootstock 
seedlings  in  the  nursery,  and  of  large 
trees  in  the  grove. 


[45] 


Gravatt  (1954)1  has  recently  called 
attention  to  the  fact  that  Phytophthora 
cinnamomi  is  an  introduced  danger  to 
native  stands  of  chestnut,  shortleaf  pine, 
Douglas  fir,  and  Port  Orford  cedar,  as 


well  as  many  cultivated  plants,  and  that 
it  is  widely  spread  with  nursery  stock. 
Control  procedures  should  be  employed 
that  eliminate  these  fungi,  rather  than 
those  that  temporarily  suppress  them. 


OTHER  ORGANISMS  THAT  CAUSE  NURSERY  DISEASES 


Other  organisms  than  Rhizoctonia  and 
the  water  molds  may  sometimes  cause 
diseases  of  nursery  crops.  In  addition, 
some  other  seedling  diseases,  such  as 
downy  mildew  of  snapdragon  or  bac- 
terial leaf  spot  of  delphinium,  may  be 
confused  with  damping-off.  It  is  possible 
to  discuss  here  only  representatives  of 
such  seedling  diseases.  Because  of  the 
difficulty  of  distinguishing  some  of  these 
less  common  troubles,  the  grower  should, 
when  in  doubt,  consult  the  local  farm 
advisor.  The  policy,  already  discussed, 
of  eliminating  the  pathogen  should  be 
adopted  whenever  possible. 

Gray  mold 

Under  certain  circumstances  Botrytis 
cinerea  may  cause  losses  in  flats  of  a 
wide  variety  of  plants.  The  fungus  is  able 
to  infect  only  through  dead  or  dying 
plant  parts  under  continued  cool,  moist 
conditions,  and  works  from  the  top 
downward.  It  starts  in  (1)  seedlings  in- 
jured by  other  causes,  such  as  fertilizer 
or  salinity  burn  or  water  dripping  from 
the  greenhouse,  or  (2)  from  foreign 
plant  parts  (for  example,  petals)  that 
have  fallen  on  the  seedlings.  The  fungus 
spreads  to  adjacent  seedlings,  which 
finally  become  covered  with  a  woolly 
gray  growth  (fig.  33).  This  fungus  can 
be  identified  by  this  growth,  by  its  re- 
striction to  cool,  moist  conditions,  and 
by  the  fact  that  it  begins  with  injured  or 
dead  parts. 

'  Citations  given  in  the  lex!  by  author  and 
•  late    will    be    found,    listed    by    sections,    under 

"References"  in  the  Appendix. 


It  is  possible  to  prevent  this  disease 
by  growing  seedlings  under  either  drier 
or  warmer  conditions  and  avoiding  in- 
juries to  the  plants,  despite  the  fact  that 
the  fungus  is  extremely  common  and  is 
air-borne. 

The  cottony-rot  fungi 

Sclerotinia  sclerotiorum  and  S.  minor 
sometimes  cause  damping-off  of  seed- 
lings. They  attack  healthy  plants  and 
under  cool,  moist  conditions  spread  very 
rapidly  through  the  flat  or  seedbed.  They 
are  easily  recognized  by  the  dense,  white, 
cottony  growth  in  which  are  found 
numerous  hard  black  sclerotia  (fig.  34). 
These  fungi  most  commonly  occur  as  soil 
organisms,  being  spread  and  controlled 
in  much  the  same  way  as  Rhizoctonia. 
The  sclerotia  may  also  sometimes  be 
spread  with  the  seed.  A  shooting-spore 
stage  is  rarely  formed  under  greenhouse 
conditions,  so  the  air-borne  spores  may 
be  ignored  there.  Under  field  conditions, 
however,  they  are  rather  commonly  de- 
veloped, and  infection  by  the  air-borne 
spores  gives  rise  to  the  white  blight  of 
aerial  parts  of  some  crops  (for  example, 
stock,  petunia). 

The  aster-wilt  fungus 

Fusarium  oxysporum  f.  callistephi 
may  cause  damping-off  in  seed  flats  or 
beds.  It  attacks  no  other  plant  than  the 
China  aster  and  causes  damping-off  only 
in  very  heavily  infested  or  warm  soil. 
The  seedling  rots  from  the  root  upward 
and  falls  over,  without  showing  any 
fungus     growth.     Recognition     of     this 


[46] 


trouble  may  require  the  assistance  of  the 
farm  advisor,  but  it  should  be  suspected 
if  losses  are  restricted  to  aster,  if  the  soil 
is  known  to  be  infested  with  aster  wilt 
and  has  been  held  at  temperatures  above 
70°  F,  or  if  untreated  seed  has  been 
planted.  Control  of  this  disease  is  by  soil 
and  seed  treatment. 

Nematodes 

These  animals  may  cause  (1)  swell- 
ings on  the  roots  (root-knot  nematodes, 
Meloidogyne  spp.) ,  (2)  roots  to  be  killed 
or  lesions  produced  on  them  (Pratylen- 
chus  spp.  and  many  other  surface-feed- 
ing types),  (3)  stem  enlargement  or 
necrosis  (stem  and  bulb  nematode,  Dity- 
lenchus  spp.),  and  (4)  dead  areas  in 
leaves  (foliar  nematodes,  Aphelenchoides 
spp.).  They  are  tiny,  colorless,  wormlike 
animals  barely  visible  to  the  naked  eye, 
and  are  spread  in  the  ways  already  de- 
scribed for  Rhizoctonia.  There  are  many 
types  of  soil-inhabiting  nematodes,  and 
a  great  deal  of  specialized  information 
has  been  published  about  them.  For  our 
purposes  here,  however,  it  is  important 
that  the  same  general  control  procedures 
(clean  soil — see  sees.  8  and  11;  clean 
stock — see  Sec.  13;  sanitary  practices — 
see  sees.  1,  12,  and  14,  as  well  as  this 
one)  outlined  for  fungi  in  this  manual 
will  prevent  their  damage. 

Other  seedling  diseases 

Other  diseases  are  frequently  confused 
with  damping-off.  For  example,  the 
downy  mildew  ( Peronospora  antirrhini) 
of  snapdragon  may  attack  the  leaves  and 
stems  of  seedlings,  killing  them  to  the 
ground.  The  leaves  have  a  dull  green 
color,  become  rolled,  and  have  a  dense, 
dirty  white,  mealy  fungus  mass  on  the 
undersurfaces.  Seedlings  killed  to  the 
ground  frequently  will  resprout  and  de- 
velop new  tops,  a  condition  that  does  not 
occur  with  damping-off.  The  disease  oc- 
curs only  under  very  moist  cool  condi- 
tions, and  may  be  prevented  by  growing 
in  a  drier  or  warmer  greenhouse.  The 


application  of  a  Parzate  dust  has  proved 
effective  in  control. 

The  bacterial  leafspot  of  delphinium 
(caused  by  Pseud  omonas  delphinii) 
sometimes  kills  seedlings.  Infections  start 
as  tiny  watersoaked  spots  in  the  leaves 
that  may  become  black  or,  under  very 
moist  conditions,  may  spread  through  the 
whole  plant.  The  disease  spreads  down- 
ward from  the  top,  and  is  thus  distin- 
guished from  damping-off.  If  the  disease 
is  controlled  by  a  spray  of  dilute  Bor- 
deaux mixture,  the  seedlings  will  sprout 
again. 


[47] 


CONTROL  OF  NURSERY  DISEASES 


Disease  organisms  may  be  introduced 
into  a  clean  planting  through  the  medium 
of  (1)  the  soil,  (2)  the  seeds,  cuttings, 
or  other  planting  material,  and  (3)  bits 
of  soil  on  such  things  as  tools,  flats,  and 
hoses,  or  splashed  by  water. 

Damping-off  and  related  nursery 
diseases  are  most  effectively  combated 
when  preventive  treatment  aims  at 
elimination  of  the  fungus  from  the  above 
sources  before  beginning  the  given  oper- 
ation. Such  a  complete  program  demands 
foresight  and  planning,  but  has  been 
successfully  adopted  in  several  nurseries. 

Control  Measures  Needed 

Treat  soil 

Steam  or  chemically  treat  all  soil 
mixes  and  propagating  media  to  destroy 
disease  microorganisms  and  weed  seeds 
in  them.  This  preferably  should  be  done 
in  the  container  (for  example,  flat  or 
bed)  where  it  is  to  be  used.  See  sections 
8  and  11  for  details  of  methods. 

Use  pathogen-free  seed 
and  planting  stock 

There  is  no  point  in  planting  infected 
stock  in  treated  soil,  nor  is  there  any 
excuse  for  planting  healthy  stock  in  in- 
fested soil.  Seed  may  be  heat-  or  chemi- 
cally treated  to  free  it  of  pathogens. 
Vegetative  propagative  material  can  be 
freed  of  disease  organisms  in  various 
ways,  and  such  stock  is  already  available 
for  a  number  of  commercial  crops.  See 
Section  13  for  details. 

Follow   a   sound 
sanitation   program 

To  prevent  contamination  of  the  clean 
plant  in  treated  soil  it  is  necessary  to 
disinfest  tools,  flats,  and  other  con- 
tainers (sees.  1  and  12).  Handling  prac- 
tices    that     spread     disease    organisms 


should  be  discontinued  or  modified  (see 

'The  Rhizoctonia  Story,"  above).  More 

direct  ways  for  coping  with  this  recon- 

tamination    problem    are    under    study 

(Sec.  14). 

Spot  treatments  for 
limited  infestations 

When  a  small  area  of  seedling  disease 
appears  in  a  valuable  bed  or  flat,  its 
spread  may  be  stopped  by  application  of 
a  chemical  drench  (Sec.  11)  and  the  un- 
infested  plants  saved.  In  all  stock  that  is 
to  be  grown  on  for  extended  periods  or 
is  to  be  planted  in  uninfested  field  soil, 
this  method  is  beset  with  many  hazards, 
as  already  explained.  The  most  intelli- 
gent use  of  spot  treatments  is  for  confin- 
ing disease  to  a  specific  area  in  a  grow- 
ing crop  as  a  means  of  reducing  loss, 
particularly  when  the  land  in  which  the 
seedlings  are  planted  is  to  be  subse- 
quently treated  to  destroy  the  infestation. 

Relation   to 
Certification   Programs 

In  California,  nurseries  which  meet 
the  high  standards  of  freedom  from  dis- 
eases and  pests  required  by  Rules  of  the 
State  Director  of  Agriculture  may  use 
intercounty  nursery  stock  certificates 
("pinto"  tags)  on  shipments  within  the 
state.  Shipments  bearing  these  certifi- 
cates need  not  be  held  for  inspection  at 
destination,  as  would  otherwise  be  neces- 
sary. One  of  the  requirements  of  the 
Director  is  that  certain  plants  must  be 
grown  in  treated  soil  and  adequately 
protected  from  reinfestation  in  order  to 
be  eligible  for  movement  in  shipments 
bearing  such  certificates. 

In  effect,  these  certificates  provide  an 
indication  that  the  nursery  is  using  ap- 
proved disease-prevention  practices.  The 
program  is  voluntary,  but  high  standards 
must  be  maintained  if  the  certification  is 


[48] 


to  be  kept.  Many  nurseries  treat  all  soil 
used,  whether  or  not  it  is  required,  and 
this  permits  continued  use  of  the  certi- 
ficates. Clean  soil  is  important  in  any 
certification  program. 

Benefits  from  Elimination  of 
Diseases 

When  a  disease  is  eliminated  from  a 
nursery,  production  becomes  easier, 
more  certain,  and  less  expensive. 

Increases  growth   potential 
of  crop 

Many  of  the  "secrets"  of  growing 
various  crops  are  simply  practices,  ar- 
rived at  by  costly  trial  and  error,  which 
enable  a  grower  to  live  with  a  disease. 
In  almost  all  cases,  however,  the  produc- 
tion would  be  improved  if  the  disease 
were  not  present,  because  of  the  in- 
creased growth  potentialities  of  the  crop. 
Thus,  V  erticillium  wilt  of  chrysanthe- 
mum can  be  controlled  by  using  resistant 
varieties,  but  more  and  in  some  cases 
better  varieties  are  available  if  this  is  not 
the  determining  factor.  Control  of  this 
disease  has  in  this  way  undoubtedly 
speeded  the  adoption  of  year-round  pro- 
duction. Some  growers  have  been  able 
to  produce  susceptible  varieties  in  in- 
fested soil  by  very  careful  and  costly 
watering,  but  are  now  adopting  the 
preferable  soil  treatments.  Similarly, 
losses  from  Phytophthora  root  rot  of 
heather  can  be  reduced  by  minimal 
watering,  but  plant  growth  is  retarded 
and  more  skill  is  required  in  watering 
than  when  the  disease  is  absent. 


Increases  environmental 
tolerance  of  crop 

The  presence  of  a  disease  often  dras- 
tically restricts  the  range  of  variation  in 
some  environmental  factor  that  is  toler- 
ated by  a  crop.  Elimination  of  the  disease 
in  such  cases  therefore  permits  use  of 
the  full  growth  range  of  the  crop,  and 


makes  for  easier,  less  restricted  culture. 
In  southern  California  nurseries,  the 
problems  of  salinity  and  damping-off 
illustrate  the  confusion  which  often 
arises  when  disease  prevention  is  at- 
tempted through  controlling  the  soil 
moisture  (fig.  35).  In  the  absence  of 
salinity  and  of  parasites,  seedlings  grow 
over  a  wide  moisture  range.  If  salinity 
exists,  the  soil  should  be  kept  moist  (Sec. 
4)  and,  if  water  molds  are  present,  it 
should  never  be  watered  excessively.  The 
presence  of  Rhizoctonia  complicates  the 
situation,  since  it  generally  is  favored  by 
intermediate  moisture  levels,  the  same  as 
the  seedlings.  Because  uncertainty  fre- 
quently exists  as  to  the  exact  problem  or 
problems  involved,  it  is  understandable 
how  confusion  has  arisen  from  attempts 
to  solve  these  problems  by  water  control. 
The  only  real  answer  to  this  complex 
situation  is  the  elimination  of  disease, 
rather  than  trying  to  "live  with  it". 

A  similar  difficulty  involves  the  pro- 
duction of  China  asters  (fig.  36).  Fusa- 
rium  wilt  is  favored  by  soil  temperatures 
of  60°  to  85°  F,  and  most  favored  at 
about  75°  to  80°.  Growers  who  at- 
tempt to  reduce  wilt  losses  by  growing  in 
the  cool  season  or  along  the  coast  have 
sustained  heavy  losses  from  Botrytis  and 
Rhizoctonia  crown  rot.  The  plants  re- 
main in  the  rosette  stage  under  short-day 
conditions,  the  shaded  lower  leaves  die, 
and  moisture  is  favorable  for  Botrytis 
and  Rhizoctonia.  The  best  answer  is  to 
eliminate  Fusarium  and  Rhizoctonia, 
and  to  grow  the  plants  during  warm 
weather  and  long-day  conditions. 

Benefits  are  greatest 
with  best  culture 


The  better  the  culture  of  a  crop  the 
greater  will  be  the  benefit  from  elimina- 
tion of  disease,  because  each  healthy 
plant  will  produce  greater  financial  re- 
turn than  under  poor  culture.  For  this 
reason  new  methods  of  disease  control 
are  usually  taken  up  first  by,  and  prove 
most  beneficial  to  the  better  growers. 

[49] 


No  parasites  or  salinity 


Salinity  ,  Rhizoctonia  ,  Water  molds 


Salinity  only 


O 

O 


z 
< 


Salinity,  water  molds 


Water  molds  only 


Salinity,    Rhizoctonia 


Rhizoctonia  only 


Water  molds,  Rhizoctonia 


Dry 


Moist 


Dry 


Moist 


SOIL  MOISTURE 


SOIL  MOISTURE 


Fig.  35.  Diagrams  illustrating  some  of  the  difficulties  of  preventing  damping-off  and  salinity 
in  seedling  production  through  controlling  the  soil  moisture.  The  shaded  areas  indicated  the  soil- 
moisture  levels  which  result  in  best  growth  because  of  least  damage  from  salinity  and  damping- 
off  due  to  Rhizoctonia  or  water  molds,  separately  and  in  various  combinations.  Each  of  these 
three  factors  restricts  the  levels  of  moisture  that  may  be  maintained  with  safety,  and  the  presence 
of  all  three  essentially  precludes  useful  control  in  this  way. 


No  parasites 


O 

C£ 
O 


Fusarium  wilt  only 


^rsj 


Botrytis  crown  rot  only 


Fusarium  and    Botrytis 


Fig.  36.  Diagrams  illustrating  some  of  the 
difficulties  of  preventing  Fusarium  wilt  of  China 
aster  in  southern  California  by  planting  in  the 
cool  winter  season.  The  shaded  areas  show  the 
soil  temperatures  that  result  in  best  growth  be- 
cause there  is  least  damage  from  the  indicated 
diseases.  Each  disease  restricts  the  range  of 
temperatures  at  which  the  crop  can  be  grown. 


40  50  60  70  80  90 

SOIL  TEMPERATURE  (  F) 


[50] 


Makes  possible  the  evaluation 
of  cultural  practices 

A  diseased  plant  cannot  grow  as  well 
as  its  environment  permits.  This  may  be, 
for  example,  because  a  deficient  root 
system  restricts  absorption,  a  deficient 
leaf  area  reduces  carbohydrate  forma- 
tion, or  because  an  injured  stem  impedes 
movement  of  water,  nutrients,  and  foods 
between  them.  For  these  reasons,  a  dis- 
eased plant  gives  little  indication  of  the 
growth  potentiality  of  its  performance  if 
it  were  healthy.  The  only  true  indicator 
of  the  value  of  any  given  cultural  practice 
is  provided  by  a  healthy  plant  with  a 
sound  root  system  (Sec.  5). 

Plants  with  defective  roots  may  show 
responses  to  fertilizer  application  rang- 
ing from  none  to  nearly  normal,  depend- 
ing on  the  degree  of  root  damage.  Fer- 
tilizer trials  with  plants  of  this  nature 
usually  show  no  gains,  whereas  healthy 
plants  would  have  benefited  from  them. 
It  is  difficult,  therefore,  for  a  grower  to 
determine,  through  experience,  the  best 
fertilizer  procedure  for  a  crop  unless  he 
deals  with  reasonably  healthy  plants. 

Similarly,  nutritional  investigations 
that  use  infested  stock  (even  though  it 
appears  clean),  untreated  soil,  or  both, 
may  provide  no  valid  indication  of  the 
potential  response  of  the  crop.  Nutri- 
tional research  should  be  conducted  with 
healthy  plants  in  order  to  be  generally 
applicable. 

It  is  not  necessary  for  a  large  part  of 
the  root  system  to  be  injured  to  produce 
serious  effects.  Plants  sometimes  show 
severe  injury  from  the  mere  loss  of  the 
young  white  root  tips,  sometimes  re- 
ferred to  by  growers  as  loss  of  "root 
action."  This  is  because  most  of  the  ab- 
sorption occurs  in  that  zone  of  the  roots. 
Furthermore,  the  combining  of  the  nitro- 
gen absorbed  by  the  roots,  and  the  carbo- 
hydrates   formed    by    the    leaves,    into 


amino  acids  may  occur  in  the  roots. 
These  acids  are  later  used  in  forming  the 
proteins  of  the  plant.  Perhaps  growth- 
regulating  substances  are  also  formed  in 
the  roots  (Jackson,  1956).  The  whole 
plant  is  thus  seriously  affected  by  partial 
loss  of  roots. 

If  studies  on  irrigation  practices  are 
conducted  with  plants  that  have  diseased 
roots,  an  entirely  erroneous  idea  of  the 
water  requirements  of  the  crop  will  be 
obtained.  Furthermore,  plants  with  in- 
jured roots  may  wilt  when  exposed  to 
light  of  an  intensity  necessary  for  ade- 
quate carbohydrate  formation  in  the 
leaves.  If  there  is  insufficient  carbohy- 
drate formed  and  conducted  to  the  roots 
they  will  be  further  weakened.  Light  re- 
quirements may  also  be  determined  only 
on  healthy  plants. 

Reduces  cost  of 
other  disease  controls 

The  use  of  disease-free  stock  fre- 
quently reduces  the  cost  of  other  disease- 
control  procedures.  Thus,  a  California 
celery  grower  found  that  seedlings  grown 
from  hot-water-treated  seed  in  steamed 
soil  were  free  from  late  blight,  and  when 
planted  in  the  field  required  spraying 
only  toward  the  end  of  the  season,  where- 
as plants  grown  from  untreated  seed  had 
to  be  sprayed  throughout  the  season. 

Reduces  danger  of 
disease   panics 

Periodic  disease  panics,  such  as  those 
concerning  rose  mosaic  in  1929-1932 
and  chrysanthemum  virus  stunt  in  1947- 
1950,  would  be  considerably  reduced  if 
disease-free  stock  were  more  generally 
used.  Knowledge  is  the  best  defense 
against  these  disturbing  upheavals,  par- 
ticularly when  it  is  put  into  practical  use. 

For  additional  benefits  see  sections  2 
and  16. 


[51] 


SECTION 


The  Salinity  Problem 
in  Nurseries 


Warren  R.  Schoonover 
R.  H.  Sciaroni 


How  the  problem  of  salinity  arises 

Salt  injury  to  ornamental  plants 

Detection  of  salts  in  the  soil  moisture 

What  can  be  done  about  salinity 

The  U.C.-type  soil  mixes  and  the  salinity  problem 


R 


Iurserymen  are  increasingly  aware 
of  the  fact  that  excess  soluble  salts  in  the 
root  zone  have  been  responsible  for  the 
failures  of  many  ornamental  and  horti- 
cultural plants.  Excess  soluble  salts  in  the 
soil,  known  as  salinity,  are  not  confined 
to  nurseries,  but  are  quite  common  in 
soils  of  arid  and  semi-arid  regions. 
Studies  of  the  salinity  problem  with  agri- 
cultural crops  in  the  laboratory  and  field 
have  yielded  much  information  that  is 
directly  applicable  to  the  solution  of  the 
problem  in  nurseries.  This  section  has 
been  prepared  so  that  growers  will  have 
a  better  understanding  of  the  salinity 
problem  and  what  can  be  done  about  it. 


What  are  salts? 

Salts  are  chemical  compounds  consist- 
ing of  an  acid  part,  or  ion,  and  a  basic 
part,  or  ion.  For  example,  common  table 
salt — sodium  chloride — consists  of  one 
acid-forming  ion,  chloride,  and  one  basic 
ion,  sodium.  The  two  combine  in  chemi- 
cally equivalent  quantities  to  form  a 
neutral  salt.  Some  common  acidic  ions 
are  sulfate,  nitrate,  phosphate,  and  bi- 
carbonate. Common  basic  ions  are  cal- 
cium, magnesium,  potassium,  sodium, 
and  ammonium.  Any  basic  ion  may  com- 
bine with  any  acidic  ion,  a  great  variety 
of  salts  thus  being  formed. 


HOW  THE  PROBLEM  OF  SALINITY  ARISES 


All  nutrients  needed  for  plant  growth 
arc;  absorbed  by  the  plants  in  the  form 
of  salts  or  their  ions.  Some  salts  contain 
no  plant  nutrients,  others  contain  nu- 
trients essential  to  the  plants,  and  are 
Ix-ncficial    in   proper  amounts.  All  salts, 


however,  are  harmful  beyond  the  small 
quantity  needed  for  growth.  The  prob- 
lem arises  when  the  concentration  of 
soluble  salts  in  the  soil  moisture  reaches 
levels  that  are  harmful.  Salts  may  come 
from  fertilizers,  water,  or  soil. 


[52] 


Excess  chemical  and 
organic  fertilizers 

In  nursery  soils  the  origin  of  harmful 
salts  is  most  frequently  from  chemical 
and  organic  fertilizers  which  have  been 
used  in  excess  (fig.  40).  Chemical  fer- 
tilizers such  as  ammonium  sulfate,  am- 
monium nitrate,  and  potassium  sulfate 
are  already  in  the  form  of  salts  soluble 
in  water.  Organic  materials  such  as  dried 
blood,  hoof  and  horn  meal,  and  leaf  mold 
become  mineralized  through  decay 
processes,  and  the  nutrients  are  finally 
converted  into  salts.  Nitrogen  in  the 
organic  form  (for  example,  in  hoof  and 
horn  meal,  manure,  and  leaf  mold) 
breaks  down  slowly  under  cool  condi- 
tions (Sec.  7).  Growers  may  add  such 
a  fertilizer,  and  when  it  does  not  become 
available,  add  some  quickly  available  in- 
organic nitrogen.  With  return  of  warm 
weather,  accelerated  decay  processes  may 
produce  a  sudden  excess  of  water-soluble 
nitrogenous  compounds  which  will  cause 
salinity  injury. 

Improper  irrigation  practices 

Irrigation  without  proper  attention  to 
leaching  may  cause  a  build-up  of  salts  in 
the  root  zone.  Practically  all  irrigation 
waters  contain  salts,  sometimes  in  in- 
jurious amounts  (fig.  38).  This  accumu- 
lation, added  to  that  from  fertilizers,  may 
result  ,  in  dangerous  levels  if  periodic 
leaching  is  not  practiced.  Injury  is  ag- 
gravated by  permitting  a  saline  soil  to 
become  somewhat  dry  (fig.  44).  There 
is  evidence  that  salinity  injury  may  occur 
on  some  plants,  such  as  begonias,  from 
the  sprinkling  of  saline  water  directly  on 
the  foliage  (fig.  39).  In  addition,  steam 
and  chemical  soil  sterilization  may  bring 
about  the  release  of  certain  chemicals, 
such  as  ammonium,  at  levels  toxic  to 
plants;  this  topic  is  discussed  further  in 
sees.  6,  7,  and  14. 


Poor  drainage  and 

soils  initially  high  in  salts 

The  use  of  soils  with  poor  drainage, 
especially  those  which  already  have  a 
high  content  of  soluble  salts  (fig.  37)  is 
hazardous.  Poor  drainage  restricts  leach- 
ing and  may  therefore  lead  to  salt  ac- 
cumulation. 

On  several  occasions  serious  financial 
losses  have  been  incurred  when  soils, 
sedge  peat,  leaf  mold,  compost,  manure, 
and  similar  materials,  high  in  content  of 
soluble  salts,  were  used  in  growing  mix- 
tures. Some  animal  manures  may  be 
particularly  dangerous.  In  most  feedlots, 
manure  is  scraped  up,  dried,  and  sold. 
Many  feedlots  use  salt-grain  mixtures  to 
fatten  livestock.  This  salt,  added  to  the 
urine  accumulated  in  surface  manure  by 
the  evaporation  of  water,  may  result  in 
extremely  high  salinity  levels  (figs.  41 
and  42).  During  the  past  few  years 
nurserymen  have  been  shifting  towards 
a  standard  soil  mix  such  as  the  U.  C. 
type.  One  of  the  reasons  for  this  shift  has 
been  the  danger  involved  in  using  many 
unreliable  ingredients  in  a  soil  mix,  some 
of  which  might  be  initially  high  in  salts. 

Clay  pots  and  salt  injury 

It  has  been  observed  that  used  clay 
pots  may  also  contribute  to  the  salinity 
problem.  This  is  because  moisture 
evaporation  on  the  outside  surface  leaves 
behind  the  soluble  minerals  in  concen- 
trated form  (fig.  45).  Roots  which  come 
in  contact  with  the  pot  may  be  injured 
and  even  killed.  It  has  also  been  shown 
that  soluble  salts  may  cause  rotting  of 
leaves  of  Saintpaulia  which  come  in  con- 
tact with  the  salt  crust  on  the  rims  of  clay 
pots.  These  problems  may  be  avoided 
by  soaking  the  pots  in  water  before  using 
them  again  (Sec.  12).  They  may  be 
eliminated  by  using  cans  or  plastic  con- 
tainers. 


[53] 


37 


40 


38 


41 


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39 


42 


SOURCES  OF  SALINITY 
Fig.  37.  Salts  in  the  soil  itself,  many  of  them  used  by  plants.  Water  moves  to  the  surface  carry- 
ing salts,  and  evaporates,  depositing  them  on  the  surface.  Fig.  38.  The  water  supply  is  a  common 
source  of  salts  in  California,  from  which  they  accumulate  in  the  soil.  Fig.  39.  Drops  of  irrigation 
water  evaporate  on  leaves,  leaving  a  salt  deposit  which  causes  local  injury  to  some  ornamentals. 
Fig.  40.  Fertilizer  applications  are  important  sources  of  salts  added  to  the  soil.  They  are  usually 
placed  near  the  crown  of  the  plant  and  the  dissolved  salts  are  then  in  the  root  zone.  Fig.  41. 
Animal  manures  from  feed  lots  often  carry  large  quantities  of  salts.  Feed  lots  are  often  located 
in  saline  depressions.  The  salts  from  the  soil  deposits  and  from  urine  accumulate  in  the  surface 
manure  because  of  water  evaporation  there.  Periodically  the  manure  is  scraped  up,  dried,  and 
sold.  Leaf  mold  may  have  similar  dangerous  accumulations  of  salts.  Fig.  42.  Application  of 
animal  manures  or  leaf  mold  to  nursery  plants  often  introduces  injurious  salts  to  the  root  zone. 
(Based  on  a  chart  by  K.  F.  Baker.) 

I  r>4  ] 


SALT  INJURY  TO  ORNAMENTAL  PLANTS 


Salinity  injury  occurs  over  a  con- 
tinuous range  extending  from  no  ap- 
parent damage  to  rapid  death.  In  the 
lower  concentrations  of  salinity  there 
may  simply  be  reduced  growth  without 
any  visible  symptoms.  With  somewhat 
higher  concentrations,  the  plant  may 
absorb  considerable  quantities  of  salt, 
which  tend  to  accumulate  at  the  leaf 
margins  or  tips  and  there  will  cause 
actual  burning  when,  through  evapora- 
tion, the  concentration  finally  reaches  a 
lethal  point  (figs.  3,  46,  54,  and  55).  Of 
course,  this  slow  attrition  may,  with  sus- 
ceptible soft-leaved  plants  or  plants 
grown  under  full  sun  and  under  dry  con- 
ditions, lead  eventually  to  death  of  the 
plant.  The  more  usual  thing,  however,  is 
to  render  it  unproductive  and  unsightly 
(figs.  53  and  58).  If  a  plant  is  trans- 
planted into  a  sufficiently  saline  soil,  it 
may  collapse  within  a  matter  of  hours. 

There  is  a  great  variation  among  types 
of  plants  as  to  salt  tolerance.  Some  plants 
(carnation)  will  tolerate  fairly  high 
concentrations  of  soluble  salts  in  the  soil 
moisture.  The  only  symptoms  may  be  a 
slight  yellowing  and  slow  decline  in 
vigor.  In  contrast,  some  plants  (gar- 
denia) are  so  sensitive  that  root  corro- 
sion and  scorching  of  the  leaves  will 
develop  shortly  after  exposure  (figs.  56 
and  57) . 

Some  saline  conditions  may  cause  con- 
siderable injury  to  the  roots  (fig.  56). 
Root  injury  due  to  salinity  may  lead  to 
chlorosis  of  foliage.  Salinity  injury  to 
the  foliage  of  plants  is  accentuated  under 
conditions  causing  high  transpiration 
water  loss;  for  example,  in  bright  sun- 
light under  hot,  dry  conditions  as  op- 
posed to  humid,  cool  conditions.  Shad- 
ing greenhouses  and  humidifying  the 
atmosphere  reduces  transpiration  and 
hence  the  rate  at  which  salt  injury  to  the 
foliage  occurs  (fig.  51). 


Salinity  may  aggravate  the  losses  from 
seedling  damping-off  (Sec.  3).  With 
stocks  in  field  plantings,  salts  accumulat- 
ing in  the  tips  of  leaves  may  increase  the 
severity  of  attack  by  the  Botrytis  gray 
mold  (fig.  3,  C) .  Nematodes  and  fungi 
may  produce  plant  symptoms  easily  con- 
fused with  salinity  injury. 

In  general,  certain  symptoms  are 
typical  of  the  injury  caused  by  excess 
soluble  salts.  All  or  part  of  the  following 
may  develop  under  conditions  of  high 
salinity  in  the  soil  moisture. 

Poor  Germination  of  Seeds 

Poor  germination  is  particularly  im- 
portant in  the  growing  of  bedding  plants 
started  from  seed.  Salts  may  build  up 
(fig.  43)  to  a  point  where  germination  is 
greatly  reduced.  Seeds  that  do  germinate 
may  produce  plants  that  are  stunted  and 
may  be  killed  suddenly. 

Many  times  the  surface  1/4-  to  1-inch 
layer  of  soil  may  accumulate  more 
soluble  salts  than  the  second  or  third 
inch.  This  situation  may  develop  as  a  re- 
sult of  overfertilizing  or  insufficient 
watering  to  induce  leaching,  or  both. 
Evaporation  from  the  surface  of  the  soil 
will  leave  salts  behind  in  a  concentrated 
form  (fig.  43) .  Therefore,  the  entire  root 
systems  of  small  seedlings  may  be  ex- 
posed to  high  salt  levels.  When  the  sur- 
face of  the  soil  is  allowed  to  dry  slightly, 
the  concentration  of  salts  in  the  soil 
moisture  increases  and  the  seedlings  die 
quickly  (compare  figs.  43  and  44). 

Injury  to  Tops  and  Roots 
of  Plants 

Plants  grown  in  containers  or  in  raised 
or  ground  benches  under  highly  saline 
conditions  may  develop  all  or  part  of  the 
following  symptoms:  plant  stunting  (fig. 
53),.  yellowing,  wilting,  or  shedding  of 
leaves  (figs.  57  and  58),  tip  or  marginal 


[55] 


43  •    '  ■ 


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45 


ACCUMULATION  AND  CONCENTRATION  OF  SALTS 
Fig.  43.  Salts  concentrate  in  soil  surface  from  evaporation  of  water  there,  leaving  a  deposit 
of  salts  in  upper  layers.  This  is  the  zone  in  which  seeds  germinate,  and  in  which  roots  of  seed- 
lings and  shallow-rooted  annuals  develop.  Fig.  44.  Salts  become  concentrated  in  soil  when  the 
water  content  reaches  low  levels.  Plant  injury  from  salinity  is  thus  aggravated  by  the  practice 
of  "growing  plants  dry."  Fig.  45.  Salts  accumulate  in  clay  pots  because  water  evaporates  from 
the  surface,  leaving  a  crust  of  salts.  With  many  soils  the  roots  are  most  abundant  next  to  the 
pot,  exposed  to  saline  conditions.  Metal  or  other  nonporous  containers  may  be  preferable  in 
California.  Fig.  46.  Salts  accumulate  in  leaves,  particularly  at  margins  and  tips,  because  water 
evaporates  from  them,  leaving  salt  accumulation  behind.  When  these  reach  toxic  levels  the 
tissue  may  be  killed.  (Based  on  a  chart  by  K.  F.  Baker.) 


I  r>6  1 


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PREVENTION  OF  SALINITY 
Fig.  47.  Rainfall  or  irrigation  with  deionized  water  will  wash  the  salts  down  or  (in  containers) 
leach  them  from  the  soil.  Because  no  other  salts  are  introduced,  this  treatment  is  very  effective. 
Fig.  48.  Leaching  soil  with  water  of  the  best  quality  available,  washes  the  salts  from  the  bottom 
of  containers.  The  poorer  the  water  used  in  irrigation,  the  greater  the  excess  over  plant  require- 
ments that  must  be  used,  and  the  more  frequently  leaching  must  be  done.  Fig.  49.  Keep  the 
soil  moist  so  that  the  salts  are  diluted  and  plant  injury  minimized.  Fig.  50.  Provide  good  drain- 
age so  that  salts  may  be  washed  away.  Keep  holes  open  in  pots  and  cans,  and  the  bottom 
cracks  in  benches;  avoid  hardpan  soils.  With  poor  drainage  and  high  water  table,  salts  remain 
in  place  and  concentrate  from  surface  evaporation.  Fig.  51 .  Provide  shade  and  high  humidity 
for  salinity-sensitive  plants,  to  reduce  water  loss  from  leaves  and  salt  accumulation  therein. 
Fig.  52.  Use  fertilizers  in  small  quantities  as  often  as  needed,  interspersed  with  liberal  watering. 
This  will  keep  salts  at  a  low  concentration.  (Based  on  a  chart  by  K.  F.  Baker). 


[57] 


\ 


Fig.  53.  Soluble-salt  in- 
jury to  camellia.  The  plant 
on  the  left  is  healthy;  the 
one  on  the  right  was  grown 
under  conditions  where 
soluble  salts  accumulated. 
Symptoms  were  wilting  and 
severe  leaf  burn  in  some 
cases. 


"~*tm.£ 


m 


Fig.  54.  Salinity  injury  to  Roosevelt  fern.  Note  the  marginal  leaf  burn  on  the  left  caused  by 
excess  soluble  salts  in  raised  benches.  Healthy  leaf  at  right. 


leaf  burn  (figs.  3,  46,  54,  and  55),  de- 
creased root  activity  and  sloughing  of 
roots  (fig.  56),  and  complete  collapse  of 
the  top  of  the  plant   (fig.  57). 

Experience  with  ornamentals  has 
shown  that  they  react  variously  to  excess 
soluble  salts.  The  symptoms  described 
below  were  observed  under  actual  grow- 
ing conditions  in  commercial  nurseries. 

Carnation,  stock, 
and  amaryllis  . .  . 

will  tolerate  relatively  saline  conditions. 
A  ^li^rht  yellowing,  particularly  of  older 

leaves,  and  slow  decline  in  \  Igor  are  typi- 


cal. Production  drops  off  slowly  as  salt 
content  increases. 

Camellia,  rhododendron,  and 
Roosevelt  and  Boston  ferns 

Tip  or  marginal  burning  of  leaves  de- 
velops (figs.  54  and  55).  There  is  also 
decreased  root  activity  and  partial  de- 
foliation (fig.  53).  When  conditions 
which  have  contributed  to  excess  soluble 
salts  are  removed,  recovery  may  be  very 
slow.  By  the  time  the  tops  show  visible 
symptoms,  considerable  damage  has  al- 
ready been  done  to  the  root  systems. 


I  58  1 


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Fig.  55.  Individual  leaflets  from  Roosevelt  fern,  showing  salinity  damage.  The  two  leaflets  on 
the  left  are  healthy;  the  others  show  various  stages  of  leaf  scorch. 


Fig.  56.  Decreased  root  growth  of  gardenia  caused  by  salt  build-up  resulting  from  too  much 
chemical  fertilizer.  The  root  system  at  the  right  is  normal. 


Fig.  57.  Excess  soluble  salts  caused  wilting  and  collapse  of  tops  of  the  three  gardenia  plants 
at  the  left.  The  healthy  plant  at  the  right  was  grown  under  conditions  of  low  salinity. 


Fig.  58.  The  azalea   plant  on  the  left  was  killed  by  excess  soluble  salts.  Poor  drainage  and 
lack  of  aeration  may  cause  similar  symptoms  on  azaleas.  The  plant  on  the  right  is  healthy. 


Cymbidium  orchids  . . . 

show  a  leaf  tipburn  and  dieback.  Cattleya 
types  become  yellow,  and  new  growth  is 
stunted. 

Gardenia  and  azalea 

Decreased  root  activity  and  sloughing 
off  of  roots  occur.  There  may  be  wilting 
of  leaves   and  sometimes  a  spectacular 


collapse  of  the  entire  top  of  the  plant 
(figs.  56,  57,  and  58).  Excess  salts  are 
often  blamed  for  these  conditions  on 
gardenia  and  azalea,  however,  when  ac- 
tually poor  drainage  and  lack  of  aeration 
are  the  real  culprits.  Azaleas  have  a  high 
requirement  for  good  aeration  and 
drainage. 


DETECTION  OF  SALTS  IN  THE  SOIL  MOISTURE 


The  Agricultural  Extension  Service, 
commercial  soil-testing  laboratories,  and 
many  growers  are  using  instruments 
called  Wheatstone  bridges  to  study  sa- 
linity problems.  Salt  solutions  conduct 
electricity  to  an  extent  approximately 
proportional  to  the  concentration,  and 
this  conductance  may  be  measured  by 
such  bridges.  Simplified  forms  of  these 
instruments    called     "Solubridges"    are 


available  on  the  market.  Two  types  are 
in  common  use,  the  RD-26  and  RD-15. 
They  measure  electrical  conductivity  in 
terms  of  reciprocal-ohms  or  mhos/cm. 
Since  the  mho/cm  is  a  large  unit,  a  scale 
reading  in  the  range  of  salinity  injurious 
to  plants  would  be  expressed  as  small 
decimals.  Therefore,  the  units  have  been 
divided  by  1,000  into  units  called  mil- 
limhos    or    by    100,000    into    unnamed 


i<>0] 


units.  The  scale  on  the  Solubridge  RD-26 
gives  readings  in  millimhos.  Readings 
obtained  on  the  RD-15  Solubridge  can 
be  converted  to  millimhos  by  dividing 
by  100. 

The  U.  S.  Regional  Salinity  Labora- 
tory, and  the  Agricultural  Extension 
Service  and  the  Department  of  Soils  and 
Plant  Nutrition,  University  of  Califor- 
nia, have  adopted  the  saturated-soil-ex- 
tract method  for  determining  the  salinity 
of  soils.  An  approximate  value  for  the 
concentration  of  soluble  salts  in  the 
saturation  extract  expressed  as  parts  per 
million  (ppm)  can  be  obtained  by  mul- 
tiplying the  reading  on  the  RD-26  by 
about  650,  and  the  RD-15  by  about  6.5. 

Brief  instructions  for  making  soil- 
salinity  measurements  using  the  extracts 
from  saturated  soils  are  given  on  p. 
62.  Detailed  directions  can  be  obtained 
from  U.  S.  Department  of  Agriculture 
Handbook  60  or  the  offices  of  the  Agri- 
cultural Extension  Service.  When  exam- 
ining soils  and  potting  mixtures,  it  is 
very  important  that  the  extract  on  which 
the  salinity  measurement  is  made  should 
be  directly  related  to  the  soil  solution 
which  is  actually  in  contact  with  the 
roots.  The  best  procedure  would  be  to 
extract  some  of  the  moisture  with  its  dis- 
solved salts  from  the  moist  soil  in  the 
root  zone  of  plants.  Unfortunately,  this 
is  too  difficult  to  be  practical.  The  next 
best  procedure  is  to  saturate  a  soil 
sample  with  distilled,  salt-free  water  and 
then  take  out  some  of  the  water  by  suc- 
tion. This  is  easily  done.  The  amount 
of  water  required  to  saturate  a  soil  is 
directly  related  to  the  amount  it  will 
hold  under  natural  conditions  of  good 
drainage. 

If  larger  amounts  of  water  are  used, 
such  as  2  parts  or  5  parts  of  water  to  1 
of  soil,  there  will  be  no  fixed  relation 
between  the  concentration  in  the  extract 


and  the  concentration  in  the  soil  solu- 
tion. The  results  will  be  meaningless 
when  comparing  one  soil  with  another, 
unless  water-holding  capacity  is  con- 
sidered. For  example,  two  greenhouse 
soils  containing  equal  amounts  of  solu- 
ble salts  were  examined.  No.  1  was  a 
sandy  soil  which  would  hold  24  per  cent 
water  when  saturated;  no.  2  was  a  finer- 
textured  soil  holding  64  per  cent  water 
when  saturated.  Saturation  extracts  both 
contained  about  6,000  ppm  of  total  salts, 
a  harmful  amount,  and  gave  conduc- 
tivity readings  of  about  8  millimhos/cm. 
When  a  suspension  of  1  part  of  soil  to 
2  parts  of  water  was  examined,  the  first 
soil  gave  a  reading  of  1.0  millimho/cm 
and  the  second  4.5.  By  ordinary  stand- 
ards, the  first  soil  would  have  been  con- 
sidered safe  and  the  second  quite  in- 
jurious. Both  were  actually  salty  to  the 
point  that  tender  plants  would  have  been 
damaged  severely. 

Unfortunately,  many  laboratories  still 
report  results  on  soil  extracts  of  varying 
soil/water  ratios  without  including  any 
data  from  which  one  might  predict 
the  probable  salinity  to  which  plant  roots 
are  exposed.  If  growers  use  the  satura- 
tion-extract method  it  will  be  easier  to 
interpret  their  results,  and  all  work  done 
will  contribute  to  a  common  pool  of 
knowledge. 

With  agricultural  crops  in  field  soils, 
it  is  pretty  well  known  how  much  total 
salinity  can  be  tolerated  and  still  secure 
reasonable  yields.  This  is  not  yet  the 
case  with  many  ornamental  and  flower 
plants.  Additional  experience  is  needed 
with  ornamentals,  but  experience  to  date 
indicates  that  little  or  no  difficulty  will 
be  encountered  if  the  concentration  of 
the  saturation  extract  is  around  2  mil- 
limhos/cm. A  concentration  somewhat 
lower  than  this  will  allow  for  ample 
quantities  of  nutrient  salts. 


[61] 


Procedure  for  Determining  Salinity 
by  Saturation  Extract  Method1 

1.  Collect  a  soil  sample  which  is  representa- 
tive of  the  root  zone. 

2.  Prepare  a  saturated  soil  paste  as  follows: 
Fill  a  pint  container  half  full  of  the  sample  to 
be  tested.  Add  distilled  water  slowly  until  the 
whole  soil  mass  appears  wet.  Mix  thoroughly 
with  a  small  stiff  spatula  or  the  handle  of  a 
spoon,  adding  more  water  or  more  soil  as  may 
be  needed  to  reach  the  saturation  point.  When 
this  point  is  reached,  the  soil  paste  should 
show  the  following  characteristics: 

A.  Will  be  somewhat  plastic  and  will  tend 
to  shift  or  flow  slightly  when  the  con- 
tainer is  tipped. 

B.  Will  slide  freely  from  the  spatula  or 
spoon,  except  in  the  case  of  a  heavy 
clay  soil  (may  not  be  true  of  mixes 
containing  a  very  high  percentage  of 
peat) . 

C.  Will  show  a  very  little  free  water  in 
surface  depressions  upon  standing  a 
few  minutes. 


1  Adapted  from  methods  in  "Diagnosis  and 
improvement  of  saline  and  alkali  soils,"  U.  S. 
Dept.  Agr.,  Agr.  Handbook  60,  1952. 


When  the  saturation  point  apparently  has 
been  reached,  allow  the  sample  to  stand  15 
minutes  or  longer,  then  restir,  and  recheck  it 
according  to  the  three  criteria  above. 

3.  Remove  an  extract  from  this  saturated  soil 
as  follows:  Set  up  a  suction  filtering  assembly 
(fig.  59;  equipment  listed  in  Appendix).  A 
convenient  assembly  consists  of  a  size  1-A  or 
size  2  Biichner  funnel  fitted  by  a  rubber  stopper 
onto  a  500-ml  side-neck  suction  (Erlenmeyer) 
flask.  It  is  advisable  to  catch  the  extract  in  a 
test  tube,  25  by  150  mm  size,  placed  within  the 
flask.  Vacuum  is  provided  by  connecting  the 
side  neck  of  the  flask  to  a  filter  pump.  Place  a 
dry  7-cm  hard  (Whatman  No.  50)  filter  paper 
in  the  clean,  dry  Biichner  funnel  and  fill  with 
the  saturated  soil  paste.  Apply  suction  and  con- 
tinue until  enough  filtrate  is  obtained  to  de- 
termine conductivity.  This  determination  will 
require  about  6  milliliters  for  rinsing  a  small 
conductivity  cell  and  making  the  test.  This 
amount  of  extract  would  fill  the  test  tube  de- 
scribed to  a  depth  of  about  1  inch.  Do  not 
continue  suction  after  the  soil  dries  and  cracks, 
and  air  starts  passing  through. 

4.  Determine  electrical  conductivity  of  the  sat- 
uration extract.  The  conductivity  measurement 
should  be  made  according  to  the  directions 
furnished  with  the  instrument  you  are  using. 


Fig.  59.  A  suction  filtering  assembly  for  making  extracts  of  saturated  soils.  A  Solubridge  is 
at  the  left.  A  filter  pump,  connected  to  a  water  faucet,  provides  a  vacuum  to  speed  the  filtering 
operation. 


! 


\p»«-.: 


If  the  reading  is  in  terms  other  than  millimhos/ 
cm,  convert  to  these  units  by  shifting  the 
decimal  point.  The  shift  most  commonly  re- 
quired is  two  places  to  the  left,  to  convert  from 
the  common  unit  EC  x  105.  The  conductivity 
cells  with  which  tests  are  made  are  not  always 
accurately  adjusted  to  give  direct  readings. 
They  can  be  tested  with   N   potassium  chloride 

solution  (see  Appendix),  which  should  give  a 
reading  of  1.41  millimhos/cm.  The  amount  of 
the  reading  above  or  below  this  figure  should 
be  added  to  or  subtracted  from  the  normal 
setting   for  the  observed   temperature   in   each 


case.  It  is  important  to  test  the  solution  at  the 
exact  temperature  for  which  the  instrument  is 
designed,  or  to  make  a  temperature  adjust- 
ment. Many  instruments  provide  a  dial  for  ad- 
justing the  temperature. 

Test  the  extract  you  have  prepared,  as  fol- 
lows: If  enough  extract  is  available,  rinse  the 
cell  twice,  discard  the  rinsings,  and  fill  the 
cell,  being  sure  there  are  no  air  bubbles.  If  in- 
sufficient extract  is  available  for  rinsing,  wa<h 
the  cell  out  with  distilled  water,  rinse  it  twice 
with  acetone,  and  take  air  in  and  out  of  the 
cell  until  it  is  dry.  Record  the  electric  con- 
ductivity reading  in  millimhos/cm. 


WHAT  CAN  BE  DONE  ABOUT  SALINITY 


The  problem  of  excess  soluble  salts 
need  not  be  troublesome  to  nurserymen. 
Dangers  of  salt  accumulations  can  be 
minimized  and  practically  eliminated  if 
proper  practices  are  followed. 

Water  Quality  and  Proper 
Irrigation  Practices 

The  use  of  proper  irrigation  practices 
is  important  in  reducing  the  dangers  of 
excess  soluble  salt  accumulation.  In  any 
type  of  irrigated  agriculture,  nursery 
stock  included,  it  is  always  necessary  to 
use  excess  water  to  bring  about  some 
leaching  (fig.  48).  In  many  nurseries, 
constant  water  level  and  subirrigation 
practices  are  used.  With  subirrigation, 
salts  accumulate  on  the  surface  of  the 
soil.  One  must  be  careful  either  to  avoid 
mixing  these  salts  with  the  deeper  soil,  or 
better  yet,  wash  them  down  completely 
through  the  bed  by  an  occasional  very 
heavy  watering.  A  light  watering  may 
merely  take  these  salt  accumulations 
down  into  the  root  zone  where  they  will 
cause  great  damage.  Large  amounts  of 
water  will  be  required  for  these  heavy 
leachings  if  salts  are  allowed  to  build  up. 
In  areas  subject  to  salinity  troubles,  sub- 
irrigation  may  be  a  dangerous  method 
for  use  by  inexperienced  operators. 

Irrigation  water  should  be  low  in  salts 
(fig.  47) .  The  Solubridge  can  be  used  to 


measure  the  conductivity  of  water  and 
thus  provide  a  measure  of  total  salts  in 
water.  A  good  water  should  give  a  read- 
ing less  than  1  millimho/cm,  and  should 
have  the  individual  constituents  present 
in  favorable  proportions.  The  conduc- 
tivity, however,  is  only  one  of  several 
factors  used  in  evaluating  the  quality  of 
irrigation  water.  It  will  be  advisable  to 
have  water  from  unknown  sources,  such 
as  wells,  analyzed  before  planning  exten- 
sive use.  When  judging  waters  on  the 
basis  of  this  item  only,  the  higher  the 
conductivity  the  greater  the  amount  of 
water  which  must  be  leached  through  the 
root  zone.  A  water  giving  a  reading  of  2 
or  more  would  be  relatively  bad  for  most 
ornamental  plants  even  with  satisfactory 
ratios  of  the  constituents.  If  used  at  all, 
it  should  be  applied  in  large  quantities 
to  bring  about  leaching  and  prevent  con- 
centration. 

Irrigation  waters,  in  addition  to  the 
danger  of  creating  salinity  problems  in 
the  soil,  may  leave  an  objectionable 
residue  on  the  foliage  of  plants,  usually 
because  of  high  calcium  or  magnesium 
bicarbonate  content  (fig.  39).  Several 
large  nurseries  are  using  "deionizing 
units"  for  removing  all  or  part  of  the 
salts  from  water.  These  units  are  expen- 
sive initially  but  where  a  serious  water- 
quality  problem  exists,  thev  may  prove 


[63] 


economical  and  practical  in  the  long  run. 
Deionized  water  does  not  have  to  be  re- 
duced to  the  level  of  distilled  water.  It 
can  be  blended  with  raw  water  to  bring 
down  the  total  salinity.  The  use  of  "de- 
ionizing  units"  or  water  sources  already 
low  in  bicarbonates  will  prevent  water 
spotting  and  therefore  eliminate  the 
added  expense  of  washing  residue  off  the 
leaves.  Leaf  absorption  of  salts  is  be- 
coming more  widely  recognized.  In 
southern  California  a  water  moderately 
high  in  sodium  but  containing  practically 
no  calcium  has  defoliated  azaleas  under 
conditions  of  sprinkler  irrigation.  Over- 
head watering  should  be  discontinued  if 
this  situation  is  confronted. 

The  presence  of  high  concentrations 
of  sodium  offers  a  special  problem  in  de- 
termining water  quality.  If  sodium  is 
high  relative  to  calcium  plus  magnesium, 
the  water  is  likely  to  cause  trouble.  This 
is  especially  true  if  the  water  is  high  in 
bicarbonate  rather  than  chloride  or  sul- 
fate. 

Another  constituent  which  causes 
trouble,  even  in  concentrations  as  low  as 
0.5  ppm,  is  boron.  Unfortunately,  de- 
ionizers  do  not  remove  boron. 

If  excess  sodium  or  boron  is  suspected, 
growers  are  urged  to  obtain  a  water 
analysis.  Every  effort  should  be  made  to 
develop  a  new  source  of  irrigation  water 
if  the  analysis  shows  that  the  present 
water  is  unsuitable. 

The  question  often  arises  as  to  why 
water -softening  processes  cannot  be  used 
to  remove  salts  from  water  for  nursery 
use.  Waters  are  called  "hard"  when  they 
contain  sufficient  quantities  of  calcium 
and  magnesium  salts  to  precipitate  soap 
to  an  undesirable  extent.  Common  water- 
softening  processes  do  not  remove  salts 
from  the  water,  they  merely  substitute 
sodium  for  equivalent  quantities  of  cal- 
cium and  magnesium.  This  produces  a 
water  which  is  more  desirable  for  laun- 
dry and  washing  purposes,  but  much  less 
satisfactory   for  irrigation. 


Soils  of  moderate  to  high  salinity 
should  be  kept  on  the  moist  side  to  re- 
duce plant  injury  (fig.  49)  ;  alternate 
wet  and  dry  conditions  should  be 
avoided.  Growing  the  plants  under  shady, 
humid  conditions  also  reduces  water  loss 
from  the  leaves,  and  salt  accumulation 
in  them   (fig.  51). 

Selection  of  Well-drained 

Soils  Low  in  Salts 

It  is  a  common  practice  for  nursery- 
men to  use  soil  mixes  in  an  effort  to  ob- 
tain so-called  well-drained  growing 
media.  Drainage  with  sufficient  leaching 
is  very  important  in  a  program  to  mini- 
mize the  salinity  problem  (fig.  50).  No 
pot  or  bed  can  be  said  to  be  really  well- 
drained,  since  the  bottom  layer  must  be- 
come saturated  before  water  will  drip 
into  the  air  space  beneath.  A  good 
medium  has  good  permeability,  which 
means  that  water  will  move  quickly 
through  the  soil  to  the  lowermost  zone 
of  saturation,  from  which  it  will  drip. 
The  selection  of  a  sandy  soil,  initially 
low  in  total  salts,  such  as  is  used  in  a 
U.  C.  system,  has  proved  practical  and 
efficient  (sees.  5  and  6).  Because  of  the 
excellent  permeability,  the  danger  of  ex- 
cess soluble  salt  accumulation  is  elimi- 
nated if  the  proper  amount  of  fertilizers 
is  used.  Further,  with  a  U.  C.-type  mix 
there  is  less  danger  of  overwatering. 

Proper  Use  of  Fertilizers 

Many  nurserymen  have  gotten  into 
the  habit  of  using  more  fertilizer  than  is 
actually  needed  to  produce  adequate 
plant  growth  and  maximum  quality  (fig. 
52) .  This  is  perhaps  due  to  the  fact  that 
there  is  little  concern  over  cost  of  ferti- 
lizer for  ornamentals,  because  of  their 
high  value.  Overfertilization  has  con- 
tributed markedly  to  the  problem  of  ex- 
cess soluble  salts. 

Laboratory  tests,  field  demonstrations, 
and  actual  commercial  usage  have  shown 
that  proper  amounts  of  fertilizer  applied 


64 


to  a  U.  C.-type  mix  produce  excellent 
plant  growth.  All  of  this  has  been 
achieved  with  practically  no  risk  from 
the  danger  of  overfertilization  and  re- 
sulting salinity  problem. 

The  fertilizer  schedules  used  in  the 
U.  C.  system  would  provide  sufficient 
plant  nutrients  with  safety  from  the 
standpoint  of  salinity  (sees.  5,  6,  and  7) . 


Frequency  of  application  of  the  fertilizer 
will  depend  upon  kind  and  size  of  plant, 
amount  of  growth,  container  size,  and 
other  factors.  Conductivity  readings  of  a 
saturation  extract  from  a  U.  C.-type  mix 
varies  from  1.5  to  3.5  millimhos/cm. 
This  is  considered  a  safe  margin  for  most 
ornamental  plants,  lower  levels  being 
preferable  for  seedlings. 


THE  U.  C.-TYPE  SOIL  MIXES  AND  THE  SALINITY  PROBLEM 


The  adoption  of  mixtures  of  fine  sand 
and  organic  matter  as  plant  growing 
media  can  help  eliminate  the  salinity 
problem  because  of  increased  perme- 
ability and  leachability  of  such  mixtures. 
In  one  of  our  experiments  to  determine 
comparative  leachability,  potting  mix- 
tures were  set  up  consisting  of  fine  sand 
alone,  sand  with  25  per  cent  and  50  per 
cent  peat,  sand  with  50  per  cent  fir  bark, 
loam  with  50  per  cent  pine  shavings, 
loam  with  50  per  cent  fir  bark,  loam  with 
50  per  cent  peat,  and  loam  treated  with 
Krillium  soil  conditioner.  All  these 
potting  soils  were  salinized  with  a  mix- 
ture of  sodium  sulfate  and  calcium  chlo- 
ride to  a  salinity  of  approximately  7.0 
millimhos/cm  in  the  saturation  extracts. 
Leaching  experiments  were  conducted  to 
study  the  rate  of  salt  removal  from  soil 
beds  1%  inches  in  depth. 

The  results  of  these  experiments  are 
diagrammed  in  figure  60.  The  vertical 
scales  indicate  the  electrical  conductivity 
of  the  leachate  passing  through  the  soil. 
The  horizontal  scales  indicate  the  depths 
of  water  passed  through  the  soil. 

These  experiments  show  (fig.  60,  A) 
that  salts  are  removed  rapidly  from  sand 
and  that  the  addition  of  organic  matter 
retards  salt  removal  even  though  it  may 
improve  permeability.  The  high  initial 
conductivity  shows  that  much  salt  is  re- 
moved in  the  first  portion  of  the  leachate. 
The  less  steep  slopes  for  the  mixes  con- 
taining peat   indicate   that  the   peat  re- 


tards salt  removal.  The  lower  initial  con- 
ductivity of  the  mix  containing  fir  bark 
indicates  that  the  fir  bark  causes  greater 
salt  retention  than  the  peat.  A  total  depth 
of  water  (2  inches),  which  was  greater 
than  the  depth  of  the  soil,  was  required 
to  reduce  the  salinity  to  reasonably  low 
levels. 

Leaching  with  a  dilute  nutrient  solu- 
tion reduced  the  salinity,  but  a  little  more 
slowly  than  leaching  with  tap  water  (fig. 
60,  B) .  The  nutrient  solution  seemed  to 
improve  permeability  of  the  sand  and 
permitted  channeling  around  the  organic 
matter  so  that  the  first  portion  of  the 
leachate  removed  less  salt  from  the  mix- 
tures containing  50  per  cent  of  either 
peat  or  fir  bark.  The  addition  of  the 
organic  materials  to  loam  evidently 
caused  channeling  which  permitted  water 
or  nutrient  solution  to  leach  through  the 
soil  without  removing  the  salt.  The  flat- 
ness of  the  curves  in  figure  60,  C,  D,  E, 
and  F  indicates  that  the  salt  is  not  being 
removed  in  the  first  portions  of  the 
leachate  as  with  the  sand  mixtures.  Tests 
indicated  that  one  third  to  one  half  of 
the  salt  remained  in  these  loam  mixes 
after  about  2  inches  of  water  had  passed 
through  the  1%-inch  layer  of  soil.  The 
Krillium  did  not  improve  the  leacha- 
bility. 

It  is  evident  that  if  there  are  sources 
of  salinity  such  as  excess  fertilizer  or 
poor-quality  irrigation  water,  leaching 
needs  to  be  frequent  and  heavy  to  com- 


[65] 


Cond. 
KxlO3 
15- 

14- 

13- 

12- 

11- 

10- 

9- 

8- 

7- 

6- 

5- 

4^ 

3- 

2- 

1- 

0- 


Tap  Water  Leaching 


Nutrient  Solution  Leaching 


Fine  Sand 

x x  Fine  Sand  +  25%  Peat 

e-—e  Fine  Sand  +  50%  Peat' 

Fine  Sand  +  50% 

Fir  Bark 


5- 
4r 

3- 
2- 
1- 
0- 


,-s. 


Loam  +  50%  Fir  Bark  1 

&—  e  Loam  +  50%  Peat 


*-o- 


T 1 


/ 


<^, 


% 


V*«L 


^«^. 


'■~o. 


T 1 


5-| 
4- 
3- 

2- 

1 
0 


_  Loam,  Krilium  Treated 
_x Loam +50%  Pine 

Shavings 


T 


T 


o'.5    l'.O  2.0  3.0 

Inches  Leachate 


4.0 


T 


T 


T 


0.5     1.0  2.0  3.0 

Inches  Leachate 


- 1 
4.0 


Fig.  60.  Results  of  leaching  experiments  with  U.  C.  and  other  soil  mixes.  The  fine  sand  was 
more  easily  leached  than  was  the  loam.  Addition  of  organic  materials  retarded  salt  removal, 
but  peat  did  this  less  than  the  other  forms  tested.  Nutrient  solutions  were  less  effective  than 
water  in  leaching.  Conductance  of  the  leachate  is  not  a  reliable  index  of  salts  remaining  in 
the  soil. 


[66] 


bat  the  salinity  problem.  This  is  more 
easily  accomplished  with  fine-sand  mix- 
tures such  as  the  U.  C.  type  than  with 
those  made  from  the  finer-textured  loam. 
The  results  also  indicate  that  measur- 
ing the  salinity  of  the  leachate  from  a 
pot  or  bed  does  not  indicate  how  much 
salt  remains  in  the  soil.  Undesirable 
quantities  of  salt  may  remain  in  soil  after 
the  leachate  has  reached  low  levels  of 


salinity.  Salinity  should,  therefore,  be  de- 
termined from  a  soil  sample.  Heavy  and 
frequent  leaching,  if  required,  will  neces- 
sitate increased  attention  to  supplies  of 
nutrients,  particularly  nitrogen. 

The  leachability  of  the  U.  C.-type 
mixes  makes  possible  the  avoidance  of 
the  salinity  problem,  but  use  of  these 
mixes  is  no  insurance  against  salinity  if 
leaching  is  inadequate. 


[67] 


SECTION 


The  U.C-Type 
Soil  Mixes 


Formulas  for  the  soil  mixes 


O.  A.  Matkin 
Philip  A.  Chandler 


Application  of  fertilizers 

Suggested  uses  of  the  U.C.  soil  mixes 


Practical  considerations 


I 


T  is  singular  that  so  many  problems 
in  nurseries  arise  from  unsuitable  grow- 
ing media,  when  there  are  so  many  types 
of  them  from  which  to  select.  Although 
many  excuses  are  offered  for  such 
failures,  the  trouble  usually  lies  in  lack 
of  understanding  of  a  few  basic  plant  re- 
quirements and  methods  of  satisfying 
them. 

Since  many  soil  mixes  have  already 
been  proposed  and  many  successful 
crops  produced  in  them,  it  may  seem  un- 
necessary to  suggest  another.  Other  sys- 
tems or  mixes,  however,  have  failed  par- 
tially or  wholly  to  satisfy  the  require- 
ments, particularly  that  of  reliability.  If 
a  grower  cannot  depend  on  a  certain  soil 

NOTE: 

Copies  of  tables  1  through  8  are  printed 
in  leaflet  form  for  ready  reference  on 
desk  or  wall.  Ask  your  farm  advisor  for 
Leaflet  89,  "The  U.C-Type  Soil  Mixes," 
or  write  to  Agricultural  Publications, 
Room  22  Giannini  Hall,  University  of 
California,  Berkeley  4,  California. 


mix  to  be  similar  in  physical  and  chemi- 
cal properties  each  time  it  is  prepared, 
he  cannot  plan  his  program  of  growing 
with  any  degree  of  certainty,  nor  learn 
from  either  his  failures  or  his  successes. 
A  system  which  provides  complete  physi- 
cal and  chemical  reliability,  along  with 
optimum  conditions  for  plant  growth, 
should  have  world-wide  value.  The  sys- 
tem proposed  is  an  attempt  to  satisfy 
this  need  in  the  simplest  manner  pos- 
sible. 

The  unique  ingredient  of  the  U.  C- 
type  mix  is  fine  sand.  This  material  has 
been  avoided  by  research  workers  be- 
cause it  was  considered  too  fine  for  sand 
culture,  and  by  growers  and  agricul- 
turalists because  of  its  low  fertility.  Sand 
culture  involves  the  use  of  coarse  sand 
or  gravel  with  practically  no  moisture  or 
nutrient  retention,  these  being  continu- 
ously supplied  by  nutrient  solution.  As 
explained  in  Section  6,  fine  sand  plus 
peat  approaches  loam  in  water  and  nu- 
trient retention,  but  avoids  the  compli- 
cations involved  where  clay  is  present. 


I  68  | 


FORMULAS  FOR  THE  SOIL  MIXES 


Physical  Ingredients- 
Basic  Mix 


-the 


The  ingredients  of  the  basic  mix  are 
fine  sand  and  peat  moss.  The  fine  sand 
must  meet  definite  specifications;  these 
are  given  in  Section  6.  Canadian  and  Ger- 
man sphagnum  peat  mosses  and  Cali- 
fornia hypnum  peat  moss  are  satisfactory 
for  the  organic  ingredient,  but  many 
black  or  sedge  peats  are  not. 

Although  there  are  but  two  basic  in- 
gredients in  the  soil  mix,  by  varying 
their  proportions  a  wide  range  of  physi- 
cal properties  is  obtainable.  Five  of  these 


variants  are  shown  in  table  1.  Other  ma- 
terials, such  as  redwood  sawdust  or  shav- 
ings, or  rice  hulls  may  be  substituted  for 
all  or  part  of  the  peat  moss,  provided 
they  too  satisfy  the  physical  and  chemical 
requirements  already  mentioned  and  ex- 
plained in  more  detail  in  Section  6.  Pos- 
sible variations  within  the  basic  prin- 
ciple are  therefore  numerous. 

Chemical  Ingredients 

Formulas  for  fertilizer  addition  to  the 
basic  mix  may  also  be  quite  numerous, 
but  they  too  follow  a  simple  basic  pat- 


Table  1.  The  Five  Basic  U.  C.  Soil  Mixes 


Ingredients,  per 
cent  by  volume 

Weight,  pounds 
per  cubic  foot 

Maximum  water 
content  f 

pH  with 
fer- 
tilizer 
added 

Approx. 
cost  per 

cubic 

yard 

Soil 
mix 

Fine 
sand 

Peat 
moss* 

At  max. 

water 
con- 
tent f 

Oven- 
dry 

Per 

cent 

by 

volume 

Per 

cent 

by 

weight 

Comments  and  suggested  uses 

A 

100 

0 

117 

89 

43 

30 

7.0 

$2.00 

Seldom    used;    densest 
and  least  retentive  of 
nutrients;  for  cans, 
flats,  beds 

B 

75 

25 

105 

76 

46 

38 

6.8 

3.19 

Commonly  used ;  good 
physical  properties ;  for 
cans,  flats,  beds 

C 

50 

50 

94 

63 

48 

48 

6.5 

4.37 

Commonly  used;  excel- 
lent physical  proper- 
ties ;  for  pots  and  beds 

D 

25 

75 

66 

34 

51 

94 

6.0 

5.56 

Light  weight,  excellent 
aeration;  for  pots  and 
beds 

E 

0 

100* 

43 

7 

59 

530 

5.7 

$6.75 

Very  light  weight;  used 
for  azaleas,  sometimes 
gardenias  and  camel- 
lias 

*  Redwood  shavings  may  be  used  for  part  of  the  peat  in  mix  E  to  improve  aeration  and  reduce  cost. 
Redwood  shavings  or  sawdust  or  rice  hulls  may  also  be  used  for  some  or  all  of  the  peat  in  other  mixes. 

f  Maximum  water  content,  and  weight  at  that  moisture  level,  are  typical  for  a  6-inch  column  of  a  mixture 
of  fine  sand  of  the  Oakley  series  and  Canadian  peat  moss. 


[69] 


tern.  The  fertilizer  ingredients  are  nor- 
mally added  by  weight  to  a  volume  of 
physical  mix.  Mixing  the  physical  in- 
gredients and  fertilizers  can  be  carried 
out  in  many  ways.  It  is  desirable  to 
establish  accurately  the  volume  of  any 
equipment  used  for  measuring  the  physi- 
cal ingredients.  Uniformity  of  filling 
measuring  equipment  with  these  in- 
gredients is  essential.  This,  with  accurate 
weighing  of  fertilizer  additions,  will  in- 
sure a  consistent  mix. 

Typical  variations  of  the  fertilizer  ad- 
ditions suitable  for  the  five  examples  of 
basic  mix  for  the  U.  C.  system  are  given 
in  tables  2  through  6.  Although  these  fer- 
tilizer additions  for  the  different  mixes 
correspond  in  general,  each  is  adapted 
to  and  should  be  used  only  with  the  mix 
it  is  suggested  for;  thus  fertilizer  I  (A) 
should  be  used  only  with  mix  A,  fertilizer 
I   (B)   only  with  mix  B,  and  so  on. 

A  grower  trying  mixtures  containing 
hoof  and  horn  should  use  several  dif- 
ferent rates  to  determine  which  is  best 
for  his  conditions  of  growing. 

In  the  foregoing  formulas  lime  has 
been  added  in  two  forms,  dolomite  and 
calcium  carbonate,  in  order  to  bring  the 
pH  up  to  a  reasonable  value  and  at  the 
same  time  supply  proper  proportions  of 
calcium  and  magnesium.  If  pH  values 
other  than  those  listed  are  desired,  the 
lime  additions  may  be  altered  to  suit  the 
new  requirements.  Fine  sand  is  fre- 
quently found  to  be  slightly  acid  in  re- 
action, though  exceptions  do  occur.  In 
the  event  lime  occurs  in  the  fine  sand 
used,  the  lime  additions  may  be  reduced 
in  the  mix. 

Potassium  nitrate  is  sometimes  diffi- 
cult to  distribute  uniformly  in  the  mix 
because  it  tends  to  cake.  Many  growers 
rub  the  lumps  over  a  window  screen  just 
before  mixing  in  order  to  reduce  it  to  a 
fine  powder.  Others  dissolve  the  potas- 
sium nitrate  in  water  and  sprinkle  it  into 
the  soil  as  it  is  being  mixed. 

Fertilizers  used  in  small  amounts  are 
sometimes  difficult  to  mix  uniformly  in 


the  bulk  ingredients.  One  of  the  best 
methods  of  obtaining  uniformity  is  to 
mix  all  fertilizer  components  together 
before  adding  the  bulk  ingredients.  It 
may  even  be  helpful  to  mix  some  fine 
sand  with  them  during  this  premixing  of 
fertilizers. 

As  mentioned  in  Section  6,  phosphate 
may  be  added  as  either  single  or  double 
superphosphate.  The  formulas  in  tables 

2  through  6  list  single  superphosphate. 
If  double  is  used,  about  one  half  as 
much  is  required.  One  of  the  problems 
in  using  the  superphosphates  is  that 
some  grades  are  very  coarse  or  in  small 
lumps,  causing  uneven  distribution  in 
the  mix.  To  overcome  this  the  grower 
should  obtain  a  finely  ground  product. 
If  the  best  material  obtainable  is  still 
rather  coarse,  he  might  screen  out  the 
lumps  and  then  grind  them  to  a  powder. 
Some  growers  have  used  technical-grade 
monocalcium  phosphate  from  chemical 
supply  houses  in  order  to  obtain  a  rela- 
tively pure  powdery  product.  The  cost 
of  the  technical-grade  material  is  2  to 

3  times  as  high,  but  this  is  not  a  major 
factor  in  the  cost  of  the  mix  since  mono- 
calcium  phosphate  can  be  used  at  about 
one  third  the  rate  of  single  superphos- 
phate. 

The  amount  of  fertilizer  added  to  the 
basic  mixes,  is  largely  limited  by  the  sa- 
linity produced.  The  components  which 
contribute  most  to  the  salinity  of  the  final 
mix  are  potassium  nitrate  and  potas- 
sium sulfate.  Of  course,  the  breakdown 
products  of  the  organic  nitrogen  addition 
will  eventually  also  contribute  to  salin- 
ity. The  fertilizer  additions  listed  above 
will  result  in  a  saturation  extract  con- 
ductivity of  approximately  2.0  (Sec.  4). 
The  formulas  are,  therefore,  a  guide  to 
the  amounts  and  types  of  materials  that 
may  be  safely  added  to  any  one  of  the 
basic  mixes.  Numerous  variations  are 
possible.  A  grower  may  wish  to  increase 
or  decrease  the  amount  of  any  one  of  the 
ingredients  and  observe  the  effect  on  his 
particular  crop.  (Continued  on  p.  76) 


[70] 


Table  2.  Chemical  Ingredients  for  U.  C.  Soil  Mix  A 

(100  Per  Cent  Fine  Sand) 

Use  these  fertilizers  only  with  mix  A 

Amount  of  materials  to  be  added  to  each  cubic  yard 

These  mixes  may  be  stored  indefinitely 


Fertilizer  I  (A) 

Fertilizer  IV  (A) 

8      oz.  potassium  nitrate 

12      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^  lb.  single  superphosphate 

2^2  lb.  single  superphosphate 

13^  lb.  dolomite  lime 

.  1}^  lb.  dolomite  lime 

2M  lb.  gypsum 

23^  lb.  gypsum 

Contains    moderate    amount   of 

available 

No  available  nitrogen  included.   Will  re- 

nitrogen but  will  require  supplemental  feed- 

quire feeding  as  soon  as  planted.  Good  for 

ing  within  a  short  time.  Good 

for  rooted 

holding  plants  back.  Good  for  rooted  cut- 

cuttings and  growing-on. 

tings  and  growing-on. 

Approximate  cost  17  cents 

Approximate  cost  13  cents 

These  mixes  should  be  planted 

within  one  week  of  preparation 

Fertilizer  II  (A) 

Fertilizer  V  (A) 

23^  lb.  hoof  and  horn  or  blood  meal 

23^2  lb.  hoof  and  horn  or  blood  meal 

8      oz.  potassium  nitrate 

12      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^  lb.  single  superphosphate 

2 '  j  lb.  single  superphosphate 

1J4  lb.  dolomite  lime 

13^  lb.  dolomite  lime 

2^  lb.  gypsum 

23^  lb.  gypsum 

Contains  available  nitrogen  plus  moderate 

nitrogen   reserve.    Good   for   fast-growing 

Moderate  supply  of  reserve  nitrogen  with 

rooted  cuttings  or  transplants  and  liners. 

none  immediately  available.  Same  uses  as 

Also  used  for  potting-on. 

formula  II  (A). 

Approximate  cost  35  cents 

Approximate  cost  31  cents 

Fertilizer  III  (A) 

Fertilizer  VI  (A) 

5      lb.  hoof  and  horn  or  blood  meal 

5      lb.  hoof  and  horn  or  blood  meal 

8      oz.  potassium  nitrate 

12      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^  lb.  single  superphosphate 

1l/i  lb.  single  superphosphate 

1%  lb.  dolomite  lime 

\l/i  lb.  dolomite  lime 

2 3^  lb.  gypsum 

23^  lb.  gypsum 

Contains  available  nitrogen  plus  high  nitro- 

High nitrogen  reserve  with  none  immedi- 

gen reserve.    Good  for   potting-on    where 

ately    available.    Same    uses    as    formula 

plants   are   quite   fast   growing   or   where 

III  (A). 

small  amounts  of  soil  are  added. 

Approximate  cost  52  cents 

Approximate  cost  48  cents 

[71] 


Table  3.  Chemical  Ingredients  for  U.  C.  Soil  Mix  B 

(75  Per  Cent  Fine  Sand, 

25  Per  Cent  Peat  Moss) 

Use  these  fertilizers  only  with  mix  B 

Amount  of  materials  to  be 

>  added  to  each  cubic  yard 

These  mixes  may  be  stored  indefinitely 

Fertilizer  I  (B) 

Fertilizer  IV  (B) 

6      oz.  potassium  nitrate 

10      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^  lb.  single  superphosphate 

2^2  lb.  single  superphosphate 

43^  lb.  dolomite  lime 

4^  lb.  dolomite  lime 

134  lb.  calcium  carbonate  lime 

134  lb.  calcium  carbonate  lime 

134  lb.  gypsum 

134  lb.  gypsum 

Contains    moderate    amount    of    available 

No  available  nitrogen  included.   Will  re- 

nitrogen but  will  require  supplemental  feed- 

quire feeding  as  soon  as  planted.  Good  for 

ing  within   a  short  time.   Very  good  for 

holding  plants  back.  Same  uses  as  formula 

bedding  plants  and  can  growing. 

KB). 

Approximate  cost  20  cents 

Approximate  cost  17  cents 

These  mixes  should  be  planted 

within  one  week  of  preparation 

Fertilizer  II  (B) 

Fertilizer  V  (B) 

23^  lb.  hoof  and  horn  or  blood  meal 

23^  lb.  hoof  and  horn  or  blood  meal 

6      oz.  potassium  nitrate 

10      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^  lb.  single  superphosphate 

23^  lb.  single  superphosphate 

43^  lb.  dolomite  lime 

4.Y  lb.  dolomite  lime 

134  lb.  calcium  carbonate  lime 

134  lb.  calcium  carbonate  lime 

134  lb.  gypsum 

134  lb.  gypsum 

Contains  available  nitrogen  plus  moderate 

Moderate  supply  of  reserve  nitrogen  with 

nitrogen   reserve.    Good   for   fast-growing 

none  immediately  available.  Same  uses  as 

rooted  cuttings,  transplants,  or  liners.  Also 

formula  II  (B). 

used  for  potting-on. 

Approximate  cost  38  cents 

Approximate  cost  35  cents 

Fertilizer  III  (B) 

Fertilizer  VI  (B) 

5      lb.  hoof  and  horn  or  blood  meal 

5      lb.  hoof  and  horn  or  blood  meal 

6      oz.  potassium  nitrate 

10      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^  lb.  single  superphosphate 

23/>  lb.  single  superphosphate 

4'^  lb.  dolomite  lime 

4 Yi  lb.  dolomite  lime 

134  lb.  calcium  carbonate  lime 

134  lb.  calcium  carbonate  lime 

134  lb.  gypsum 

134  lb.  gypsum 

Contains  available  nitrogen  plus  high  nitro- 

High nitrogen  reserve  with  none  immedi- 

gen  reserve.    Good   for   potting-on   where 

ately    available.    Same    uses    as    formula 

plants  are  quite  fast  growing  or  where  small 

III  (B). 

amounts  of  added  soil  are  used. 

Approximate  cost  55  cents 

Approximate  cost  52  cents 

L72J 


Table  4.  Chemical  Ingredients  for  U.  C.  Soil  Mix  C 
(50  Per  Cent  Fine  Sand,  50  Per  Cent  Peat  Moss) 

Use  these  fertilizers  only  with  mix  C 

Amount  of  materials  to  be  added  to  each  cubic  yard 

These  mixes  may  be  stored  indefinitely 


Fertilizer  I  (C) 

Fertilizer  IV  (C) 

4      oz.  potassium  nitrate 

8      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^2  lb.  single  superphosphate 

23^  lb.  single  superphosphate 

73^  lb.  dolomite  lime 

73^  lb.  dolomite  lime 

23^2  lb.  calcium  carbonate  lime 

23^2  lb.  calcium  carbonate  lime 

Contains  moderate  amount  of  available  ni- 

No available  nitrogen  included.   Will  re- 

trogen but  will  require  supplemental  feed- 

quire feeding  as  soon  as  planted.  Good  for 

ing  within  a  short  time.  Good  for  rooted 

holding  plants  back.  Same  uses  as  formula 

cuttings    and    growing-on.    Easily    rooted 

1(C). 

cuttings  may  be  rooted  and  started  in  it. 

Approximate  cost  22  cents 

Approximate  cost  20  cents 

These  mixes  should  be  planted 

within  one  week  of  preparation 

Fertilizer  II  (C) 

Fertilizer  V  (C) 

23^  lb.  hoof  and  horn  or  blood  meal 

23^2  lb.  hoof  and  horn  or  blood  meal 

4      oz.  potassium  nitrate 

8      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^  lb.  single  superphosphate 

23^  lb.  single  superphosphate 

73^2  lb.  dolomite  lime 

73^  lb.  dolomite  lime 

23/2  lb.  calcium  carbonate  lime 

23^2  lb.  calcium  carbonate  lime 

Contains  available  nitrogen  plus  moderate 

Moderate  supply  of  reserve  nitrogen  with 

nitrogen  reserve.  Excellent  for  greenhouse 

none  immediately  available.  Same  uses  as 

pot  plants,  fast-growing  rooted  cuttings  and 

formula  II  (C). 

liners.  Very  good  for  potting-on. 

Approximate  cost  40  cents 

Approximate  cost  38  cents 

Fertilizer  III  (C) 

Fertilizer  VI  (C) 

5      lb.  hoof  and  horn  or  blood  meal 

5      lb.  hoof  and  horn  or  blood  meal 

4      oz.  potassium  nitrate 

8      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

23^2  lb.  single  superphosphate 

23/2  lb.  single  superphosphate 

73/£  lb.  dolomite  lime 

73^  lb.  dolomite  lime 

23/£  lb.  calcium  carbonate  lime 

2}/2  lb.  calcium  carbonate  lime 

Contains  available  nitrogen  plus  high  nitro- 

High nitrogen  reserve  with  none  immedi- 

gen reserve.   Good  for  potting-on  where 

ately    available.    Same    uses    as    formula 

plants  are  quite  fast  growing  or  where  small 

III  (C). 

amounts  of  soil  are  used. 

Approximate  cost  57  cents 

Approximate  cost  55  cents 

[73] 


Table  5.  Chemical  Ingredients  for  U.  C.  Soil  Mix  D 
(25  Per  Cent  Fine  Sand,  75  Per  Cent  Peat  Moss) 

Use  these  fertilizers  only  with  mix  D 

Amount  of  materials  to  be  added  to  each  cubic  yard 

These  mixes  may  be  stored  indefinitely 


Fertilizer  I  (D) 

Fertilizer  IV  (D) 

4  oz.  potassium  nitrate 

8  oz.  potassium  sulfate 

4  oz.  potassium  sulfate 

2  lb.  single  superphosphate 

2  lb.  single  superphosphate 

5  lb.  dolomite  lime 

5  lb.  dolomite  lime 

4  lb.  calcium  carbonate  lime 

4  lb.  calcium  carbonate  lime 

Contains  moderate  amount  of  available  ni- 

No available  nitrogen  included.  Will  re- 

trogen but  will  require  supplemental  feed- 

quire feeding  as  soon  as  planted.  Good  for 

ing  within  a  short  time.  Good  for 

trans - 

holding  plants  back.  Same  uses  as  formula 

planting  and  for  seed  germination. 

1(D). 

Approximate  cost  16  cents 

Approximate  cost  14  cents 

These  mixes  should  be  planted  within  one  week  of  preparation 


Fertilizer  II  (D) 

Fertilizer  V  (D) 

2 !  ■_,  lb.  hoof  and  horn  or  blood  meal 

23^  lb.  hoof  and  horn  or  blood  meal 

4      oz.  potassium  nitrate 

8      oz.  potassium  sulfate 

4      oz.  potassium  sulfate 

2      lb.  single  superphosphate 

2      lb.  single  superphosphate 

5      lb.  dolomite  lime 

5      lb.  dolomite  lime 

4     lb.  calcium  carbonate  lime 

4      lb.  calcium  carbonate  lime 

Contains  available  nitrogen  plus  moderate 

Moderate  supply  of  reserve  nitrogen  with 

nitrogen  reserve.  Very  good  for  growing-on. 

none  immediately  available.  Same  uses  as 

Reduced  water  requirement  will  enhance 

formula  II  (D). 

nitrogen   efficiency,   resulting  in  a  lower 

supplemental  feeding  requirement. 

Approximate  cost  34  cents 

Approximate  cost  32  cents 

Fertilizer  III  (D) 

Fertilizer  VI  (D) 

5  lb.  hoof  and  horn  or  blood  meal 

5  lb.  hoof  and  horn  or  blood  meal 

4  oz.  potassium  nitrate 

8  oz.  potassium  sulfate 

4  oz.  potassium  sulfate 

2  lb.  single  superphosphate 

2  lb.  single  superphosphate 

5  lb.  dolomite  lime 

5  lb.  dolomite  lime 

4  lb.  calcium  carbonate  lime 

4  lb.  calcium  carbonate  lime 

Contains  available  nitrogen  plus  high  nitro- 

High nitrogen  reserve  with  none  immedi- 

gen reserve.  Dangerous  to  use  except  for 

ately    available.    Same    uses    as    formula 

very  fast-growing  crops  owing  to  greater 

III  (D). 

efficiency  of  nitrogen  added. 

Approximate  cost  52  cents 

Approximate  cost  50  cents 

[74] 


Table  6.  Chemical  Ingredients  for  U.  C.  Soil  Mix  E 

(1 00  Per  Cent  Peat  Moss) 

Use  these  fertilizers  only  with  mix  E 

Amount  of  materials  to  be  added  to  each  cubic  yard 


These  mixes  may  be  stored  indefinitely 


Fertilizer  I  (E) 

Fertilizer  IV  (E) 

6      oz.  potassium  nitrate 

6      oz.  potassium  sulfate 

1      lb.  single  superphosphate 

1      lb.  single  superphosphate 

23^  lb.  dolomite  lime 

23^  lb.  dolomite  lime 

5      lb.  calcium  carbonate  lime 

5     lb.  calcium  carbonate  lime 

Contains    moderate    amount   of    available 

No  available  nitrogen  included.   Will  re- 

nitrogen but  will  require  supplemental  feed- 

quire feeding  as  soon  as  planted.  Good  for 

ing  within  a  short  time.  Good  for  starting 

holding  plants  back.  Same  uses  as  formula 

rooted    cuttings    and    for    potting-on    and 

1(E). 

bedding. 

Approximate  cost  15  cents 

Approximate  cost  11  cents 

These  mixes  should  be  planted  within  one  week  of  preparation 


Fertilizer  II  (E) 

Fertilizer  V  (E) 

23^  lb.  hoof  and  horn  or  blood  meal 

23^  lb.  hoof  and  horn  or  blood  meal 

6      oz.  potassium  nitrate 

6      oz.  potassium  sulfate 

1      lb.  single  superphosphate 

1      lb.  single  superphosphate 

23^  lb.  dolomite  lime 

23^  lb.  dolomite  lime 

5      lb.  calcium  carbonate  lime 

5      lb.  calcium  carbonate  lime 

Contains  available  nitrogen  plus  moderate 

Moderate  supply  of  reserve  nitrogen  with 

nitrogen  reserve.  Nitrogen  supply  should  be 

none  immediately  available.  Same  uses  as 

sufficient  for  considerable  period  of  time. 

formula  II  (E). 

Used  for  potting-on. 

Approximate  cost  33  cents 

Approximate  cost  29  cents 

Fertilizer  III  (E) 

Fertilizer  VI  (E) 

5      lb.  hoof  and  horn  or  blood  meal 

5      lb.  hoof  and  horn  or  blood  meal 

6      oz.  potassium  nitrate 

6      oz.  potassium  sulfate 

1      lb.  single  superphosphate 

1      lb.  single  superphosphate 

23^  lb.  dolomite  lime 

23^  lb.  dolomite  lime 

5      lb.  calcium  carbonate  lime 

5      lb.  calcium  carbonate  lime 

Contains  available  nitrogen  plus  high  ni- 

High nitrogen  reserve  with  none  immedi- 

trogen reserve.  Nitrogen  may  be  excessive 

ately    available.    Same    uses    as    formula 

except  where  small  amounts  of  mix  are 

m  (E). 

used  in  potting-on. 

Approximate  cost  50  cents 

Approximate  cost  46  cents 

[75] 


It  would  be  desirable  if  investigators 
and  growers  used  the  same  system  of  soil 
mixes  so  that  information  might  be  more 
readily  transmitted  between  them.  For 
example,  the  specific  method  used  to  pro- 
duce blue  hydrangeas,  as  opposed  to 
pink,  can  be  followed  by  anyone  using 
the  same  reliable  soil  system.  Here  the 


procedure  might  be  to  reduce  or  elimi- 
nate superphosphate  and  lime,  and  to 
add  some  aluminum  sulfate. 

If  long-term  crops  are  grown,  or  if 
mixes  containing  little  reserve  fertilizer 
are  used,  fertilizer  supplement  must  be 
added  as  either  liquid  or  dry  material 
during  the  growing  period. 


APPLICATION  OF  FERTILIZERS 


Dry  Fertilizers  liver   the   quantity   of  fertilizer   desired 

As  much  as  possible  it  is  desirable  to  into    each    container    by    some    simple 

use    for    dry-fertilizer    application,    ma-  trigger  mechanism.   Suggested  fertilizer 

terials  which  are  not  readily  soluble,  in  formulas   and   rates   for   containers   are 

order  to  avoid  the  danger  of  temporary  given  in  tame  i. 

excess.  Organic  nitrogen  and  the  super-  Liquid  Fertilizers 

phosphates   fall   into   this  classification.  r™                                     j                  .     .1 

£.     r                      .        r      .,.                      .,  there  are  numerous  advantages  in  the 

since  most  potassium  iertihzers  are  avail-  £  ■,.      .j   £    ..-.*           T    1         •     x 

. ,         1     -        l  1 1     r            1                  1  use   01   liquid   iertilizers.   Labor   is   tre- 
acle only  in  soluble  form,  thev  must  be  t1          j        j   1                «.i_     x  _«.«v  •• 
,       n    -i      .  1            .         aii         ill  quently  reduced  because  the  fertilizing 
handled  with  caution.  A  slowly  soluble  ,             .    i           1                            1  •     • 
„  .    ,                  .            J            .,  can  be  carried  out  during  a  normal  lrri- 
potassium  irit  has  recently  become  avail-  c     1           r              .        £      -r 
r. ,      /c,        r.     ,,        .11           .i      1  gation.  such  application  is  sate,  it  rea- 
able    (bee.  6).  Materials  that  might  be  11                :    ..                     j  a      v 

,         ,                                 r         1.      •  sonable  concentrations  are  used.  Apphca- 

used,  and  common  rates  01  application  ..  -              ,           j                    t 

'                    p       r      1     1       lif  tions  can  be  made  at  more  trequent  m- 

per  1(JU  square  feet  tor  bed  or  bench  ier-  1    •         j                        •             i 

....          ^        .  ..  tervals  in  order  to  maintain  nearly  con- 

tihzing  are  as  follows:  u  1       i       r          •               -i  u«i«*      t'u 

D  stant  levels  ot  nutrient  availability,   lhe 

Hoof  and  horn  meal 1  to  3  pounds  disadvantage  is  that  the  nutrients  added 

Blood  meal...    1  to  3  pounds  ag    R      i(j    are    algQ    more    readn       logt 

Cottonseed  meal    1  to  3  pounds  ,            1      i        1  •            A                 1                  .,, 

Castor  pomace 1  to  3  pounds  through    leaching.    Again,    the    possible 

Fish  meal 1  to  3  pounds  combinations  of  materials  are  numerous. 

Ammonium  sulfate  %  to  1  pound  Table  8  provides  a  simple  set  of  all  neces- 

Calcium   nitrate    V2  to  1  pound  sary  variants  providing  the  major  ele- 

Ammonium  nitrate   V^to1^  pound 

Single    superphosphate    2  to  4  pounds  . 

Double  superphosphate   1  to  2  pounds  lhe  most  commonly  used  liquid  for- 

Potassium  sulfate  V\  to  1  pound  mulas  are  L-2  and  L-7.  Normal  practice 

Potassium  chloride V±  to  1  pound  is  to  use  the  liquid  fertilizer  in  place  of 

Potassium  nitrate %  to  1  pound  a  reguiar  irrigation.  These  materials  can 

Potassium  frit    (Dura-K)    2  to  5  pounds  ,                ,.     ,        .  ,               .      .           ,     ,P  ■.. 

be   applied   without   rinsing  the  toliage 

Where  containers  are  to  be  fertilized,  afterward  under  all  but  extreme  condi- 

it  is  common  practice  to  use  spoon  meas-  tions  of  bright  sunlight.  Vegetable  dyes 

ures.  These  are  available  from  any  va-  may  be  added  to  fertilizer  concentrates 

riety  store  in  sizes  ranging  from  %  tea-  as  indicators  of  injection, 

spoon    up   to    I    tablespoon.   There   is   a  Concentrates  are  more  readily  made 

definite   need    for   a   more  efficient  dis-  with  hot  water  or  by  introducing  steam 

penser   which   could   be  adjusted  to  de-  while  dissolving.  The  solutions  may  be 

I  76  I 


Table  7.  Supplementary  Dry  Fertilizers  for  Container-grown  Plants 

Suggested  rates  are  for  6-inch  pots,  gallon  cans,  and  beds  or  benches  about 
8  inches  deep.  Use  proportionately  more  for  larger,  and  less  for  smaller  soil 
volumes.  The  ingredients  should  be  carefully  mixed  before  application. 

The  rates  suggested  are  substantial.  If  light  watering  is  practiced,  it  may  be 
necessary  to  reduce  the  amounts  used.  Numerous  combinations  other  than 
those  listed  may  be  found  useful  by  trial  or  soil  testing.  In  the  average  sand- 
peat  mix  the  element  required  in  greatest  quantity  is  nitrogen,  next  is  potas- 
sium, and  last  is  phosphorus. 


SUPPLYING  NITROGEN  ONLY: 

Fertilizer  VII    1  heaping  teaspoon 

1  to  3  lb.  per  100  sq.  ft. 

Hoof  and  horn  or  blood  meal 

Particularly  useful  for  first  applications  to 
plants  in  mixes  with  fertilizers  I  and  IV,  and 
for  extra  nitrogen  supplement  in  forcing. 


SUPPLYING  NITROGEN,  PHOS- 
PHORUS, AND  POTASSIUM: 

Fertilizer  VIII    2  heaping  teaspoons 

2  to  5  lb.  per  100  sq.  ft. 

4  lb.  hoof  and  horn  or  blood  meal 

4  lb.  single  superphosphate 

1  lb.  potassium  sulfate  or  chloride 

May  be  required  after  plants  have  grown 
for  some  time  in  the  same  container. 


SUPPLYING  NITROGEN  AND 
POTASSIUM: 

Fertilizer  IX  1  heaping  teaspoon 

1  to  3  lb.  per  100  sq.  ft. 

6  lb.  hoof  and  horn  or  blood  meal 
1  lb.  potassium  sulfate  or  chloride 

Most  useful  in  mixes  A,  B,  and  C,  contain- 
ing high  proportions  of  sand,  as  these  ele- 
ments are  most  rapidly  lost  through  leach- 
ing. 


SUPPLYING  NITROGEN  AND 
PHOSPHORUS: 

Fertilizer  X  2  heaping  teaspoons 

2  to  5  lb.  per  100  sq.  ft. 

1  lb.  hoof  and  horn  or  blood  meal 
1  lb.  single  superphosphate 

Most  frequently  used  in  mixes  D  and  E 
containing  very  high  proportions  of  peat 
moss,  where  the  nature  of  the  material  re- 
sults in  most  rapid  removal  of  phosphate 
from  solution  (phosphate  fixation). 


SUPPLYING  PHOSPHORUS  AND 
POTASSIUM: 

Fertilizer  XI  1  heaping  teaspoon 

1  to  3  lb.  per  100  sq.  ft. 

6  lb.  single  superphosphate 

1  lb.  potassium  sulfate  or  chloride 

Useful  where  plants  are  to  be  held  back  by 
allowing  nitrogen  deficiency  to  occur.  Well 
suited  to  legumes. 


[-!'] 


Table  8.  Liquid  Fertilizer  Formulas  for  Use  with  U.  C.  Soil  Mixes 

Where  an  applicator  is  used,  the  liquid  can  be  made  up  in  concentrated  form 

and  diluted  through   it  to  give  the  concentrations  listed.  A  dilution  ratio  of 

more  than  1:200  is  not  practical  as  these  solutions  cannot  be  made  much  more 

than  200  times  as  concentrated  as  listed.  Solutions  may  be  stored  for  extended 

periods  without  deterioration. 

Urea — 45    to   46   per   cent   nitrogen   (may   be   dangerous  to   use   if   biuret   is 

present). 

Ammonium  nitrate — 33.5  per  cent  nitrogen. 

Mono-ammonium  phosphate  (technical  grade) — 12  per  cent  nitrogen,  61.5  per 

cent  phosphate  (P  O ). 

Potassium  chloride — 60  per  cent  potash  (K..O). 

Calcium  nitrate — 15.5  per  cent  nitrogen. 

Amounts  per  100  gallons  of  water 

SUPPLYING    NITROGEN     ONLY,     EX- 
CEPT FOR  L-3  WHERE  CALCIUM  IS 
ALSO  SUPPLIED: 

L-l:  1  lb.  urea 

L-2:  1  lb.  ammonium  nitrate 

L-3 :  2  lb.  calcium  nitrate 

Particularly  useful  for  first  applications  to 
plants  in  mixes  with  fertilizers  I  and  IV, 
and  for  extra  nitrogen  supplement  in  forc- 
ing. 

SUPPLYING  NITROGEN  AND  POTAS- 
SIUM: 

L-8:  12  oz.  urea 

12  oz.  potassium  chloride 
L-9:  12  oz.  ammonium  nitrate 

12  oz.  potassium  chloride 

Most  useful  in  mixes  A,  B,  and  C  containing 
high  proportions  of  sand,  as  these  elements 
are  most  rapidly  lost  through  leaching. 

SUPPLYING    PRIMARILY    PHOSPHO- 
RUS    AND    POTASSIUM,    BUT     IN- 
CLUDING   A    SMALL    AMOUNT    OF 
NITROGEN: 

L-10:  12  oz.  mono-ammonium  phosphate 
12  oz.  potassium  chloride 

Useful  where  plants  are  to  be  held  back  by 
allowing  nitrogen  deficiency  to  occur.  Well 
suited  to  legumes. 

SUPPLYING   NITROGEN   AND   PHOS- 
PHORUS: 

L-4:  12  oz.  urea 

12  oz.  mono-ammonium  phosphate 

L-5:  12  oz.  ammonium  nitrate 

12  oz.  mono-ammonium  phosphate 

Most  frequently  used  in  mixes  D  and  E 
containing  very  high  proportions   of  peat 
moss,  where  the  nature  of  the  material  re- 
sults in  most  rapid  removal  of  phosphate 
from  solution  (phosphate  fixation). 

SUPPLYING    PRIMARILY    PHOSPHO- 
RUS,   BUT     INCLUDING    A    SMALL 
AMOUNT  OF  NITROGEN: 

L-ll:  1  lb.  mono-ammonium  phosphate 

Useful  when  phosphate  is  low. 

SUPPLYING    NITROGEN,    PHOSPHO- 
RUS, AND  POTASSIUM: 

L-6:  8  oz.  urea 

8  oz.  mono-ammonium  phosphate 
8  oz.  potassium  chloride 

L-7:  8  oz.  ammonium  nitrate 

8  oz.  mono-ammonium  phosphate 
8  oz.  potassium  chloride 

May  be  required  after  plants  have  grown 
for  some  time  in  the  same  container. 

SUPPLYING  POTASSIUM  ONLY: 

L-12:  1  lb.  potassium  chloride 
Useful  when  potassium  is  low. 

[78] 


stored  for  extended  periods  without  de- 
terioration. Examples  of  application  of 
these  formulas  are  given  below. 

Biuret   Injury 

Many  commercial  ureas  and  urea-for- 
maldehyde preparations  have  been  found 
to  contain  biuret,  a  chemical  by-product 
formed  in  the  manufacture  and  prepara- 
tion of  these  nitrogen  fertilizers.  Biuret 
is  toxic  to  most  plants,  the  typical  symp- 
toms being  stunting,  leaf  burn,  chlorosis, 


and  even  death  of  the  plant.  Unless  the 
manufacturer  labels  the  bag  or  con- 
tainer, the  only  means  of  determining 
the  biuret  content  of  the  urea  fertilizer 
is  by  analysis  or  biological  test.  Since 
this  toxic  ingredient  may  be  a  serious 
hazard  in  plant  production,  the  grower  is 
advised  to  exercise  extreme  caution  in 
the  use  of  fertilizers  containing  or  de- 
rived from  urea.  Unless  labeled  biuret- 
free,  these  materials  should  be  used  only 
after  thorough  testing  on  each  crop. 


SUGGESTED  USES  OF  THE  U.  C.  SOIL  MIXES 


The  type  of  growing  operation  will 
largely  dictate  the  choice  of  soil  prepara- 
tion and  handling.  The  following  are 
typical  procedures  for  several  types  of 
growing.  It  is  assumed  that  in  all  cases 
the  soil  mix  will  be  steamed  or  fumigated 
for  weed  and  disease  control  prior  to 
planting. 

When  a  single  growing  procedure  is 
altered  it  usually  affects  other  operations, 
and  a  general  adjustment  to  new  meth- 
ods may  be  required.  Adoption  of  U.  C- 
type  soil  mixes  is  no  exception  to  this. 
Watering  procedures  must  usually  be 
modified  for  best  results.  Some  growers 
have  found  that,  because  of  more  rapid 
crop  growth,  the  production  schedule  is 
altered.  While  planting  dates  may  need 
to  be  altered  to  accommodate  schedules, 
faster  production  will  lower  cost  and  in- 
crease volume. 

Flats 

Mix  B  will  be  used  for  most  bedding 
plants  with  the  possible  exception  of 
some  of  the  shade  plants,  such  as  be- 
gonia and  primula,  where  mix  C  might 
be  used.  Seed  flats  will  normally  be  of 
the  same  soil  preparation  as  growing 
flats. 

The  appropriate  fertilizer  I,  II,  IV,  or 
V  is  added  to  the  basic  mix.  Since  some 


[ 


danger  from  ammonium  excess  exists 
where  organic  nitrogen  is  present  in 
quantity,  many  growers  avoid  this  pos- 
sibility by  using  fertilizer  I  or  IV 
throughout.  If  organic  nitrogen  is 
omitted,  fertilizer  is  normally  applied 
soon  after  transplanting.  If  fertilizer  I  is 
used,  application  of  subsequent  material 
is  normally  delayed  1  to  2  weeks,  then  a 
broadcast  application  of  blood  or  hoof 
and  horn  meal  may  be  made,  or  a  pro- 
gram of  using  liquid  nitrogen  as  formula 
L-2  or  L-3  may  be  used  at  1-  to  2-week 
intervals.  If  plants  are  held  for  an  ex- 
tended period,  fertilizer  VIII  may  be 
used  for  broadcast  application. 

In  a  few  words,  one  procedure  would 
be: 

Physical  mix  B. 

Fertilizer  I  (B) . 

After  2  weeks  use  liquid  L-2  at 
10-day  intervals  on  transplants.  Use 
no  additional  fertilizer  on  seed  flats. 

One  week  prior  to  sale  of  trans- 
plants, apply  fertilizer  VIII  at  4 
pounds  per  100  square  feet  of  flat 
area. 

Pots 

Pot  plants  may  start  with  rooted  cut- 
tings planted  in  2%-  or  3-inch  pots 
with  one  or  more  subsequent  shifts  up- 

79] 


ward  in  pot  size  before  the  product  is 
ready  to  sell.  Many  variations  are  prac- 
ticed. Some  growers  actually  place  the 
rooted  cuttings  in  6-inch  pots,  carrying 
them  in  these  to  salable  size.  By  proper 
selection  of  fertilizer  formula  this  is  both 
possible  and  entirely  practical,  saving 
considerable  labor  in  transplanting.  The 
following  suggestions,  however,  are  in- 
tended for  the  common  transplanting 
and  shifting  procedure. 

Rooted  cuttings  will  be  placed  in  mix 
C  fortified  with  fertilizer  I  (C),  II  (C), 
IV  (C),  or  V  (C).  When  well  rooted  in 
the  new  medium,  dry  fertilizer  might  be 
applied  as  fertilizer  VII,  or  liquid  L-l, 
L-2,  or  L-3.  When  ready  for  shifting  to 
larger  pots,  the  soil  preparation  will  be 
mix  C  or  D,  with  the  appropriate  ferti- 
lizer III  or  VI.  If  the  plants  are  held  for 
a  short  period  no  further  fertilization 
should  be  required.  If  held  for  a.  long 
time,  dry  fertilizer  VIII  or  a  program  of 
liquid  fertilization  with  liquid  L-6  or 
L-7  may  be  used. 

In  brief,  one  procedure  might  be: 

Plant  rooted  cuttings  in  2V2-incn 
pots  of  mix  C,  fertilizer  I   (C). 

After  10  days  start  using  liquid 
L-2  at  10-day  intervals. 

Shift  21X>-inch  liners  up  to  6-inch 
pots  using  physical  mix  D  and  fer- 
tilizer III  (D) .  If  plants  are  held  for 
more  than  2  months  in  6-inch  pots, 
apply  1  heaping  tablespoon  fertilizer 
VIII  to  each. 

Cans 

In  can-grown  nursery  stock  the  cost  of 
the  soil  preparation  is  a  very  important 
factor,  and  in  many  cases  the  shipping 
weight  is  also  economically  important. 
For  these  reasons,  nurserymen  may  use 
materials  in  the  physical  mix  which  will 
reduce  both  cost  and  weight  of  ingredi- 
ents. Shavings,  sawdust,  bark,  and  even 
rice  hulls  are  currently  in  use  as  substi- 
tutes for  part  of  the  peat  moss.  Nursery- 
men should  acquaint  themselves  with 
these  and  other  possibilities,  keeping  in 


mind  that  any  substitute  material  must 
conform  to  the  standards  outlined  in 
Section  6  if  reliability  is  to  be  retained. 

For  this  type  of  growing,  physical 
mix  B  might  well  be  used  as  the  base, 
with  fertilizer  I  (B),  II  (B),  IV  (B),  or 
V  (B).  If  soil  is  to  be  stored  for  any 
length  of  time,  as  is  frequently  done,  the 
fertilizer  formulas  are  limited  to  I  (B) 
or  IV  (B).  The  liners  which  are  to  be 
planted  into  gallon  cans  may  be  handled 
the  same  way  as  rooted  cuttings  under 
the  discussion  of  pot  plants.  If  mix  B 
and  fertilizer  I  (B)  are  used,  either  dry 
fertilizer  VII  should  be  applied  about  2 
weeks  after  planting,  or  liquid  L-2  or 
L-3  should  be  applied  at  that  time  and 
repeated  at  approximately  10-day  in- 
tervals. After  plants  are  well  established, 
dry  fertilizer  VIII  may  be  applied  or  the 
liquid  program  shifted  to  liquid  L-6  or 
L-7.  When  plants  in  gallon  cans  are 
moved  up  to  egg  cans  (3-gallon  size)  or 
5-gallon  cans,  the  same  procedure  of  fer- 
tilizing may  be  carried  out  as  was  out- 
lined for  liners  into  gallon  cans. 

A  typical  procedure  would  be: 

Grow  liners  as  described  for  pot 
plants. 

Transplant  liners  into  gallon  cans 
using  mix  B,  fertilizer  I   (B). 

After  2  weeks  begin  applications 
of  liquid  L-2  every  third  irrigation. 

After  2%  months  shift  to  liquid 
L-7. 

Just  before  plants  are  sold,  or  in 
preparation  for  the  winter  rainy  sea- 
son (when  liquid  fertilizer  cannot 
be  applied),  use  dry  fertilizer  VIII. 

Benches  and  Beds 

Plants  grown  in  benches  or  beds 
would  include  cut  flowers  as  well  as 
stock  plants  from  which  cuttings  are  pe- 
riodically taken. 

Usually  this  type  of  growing  is  carried 
out  in  the  glasshouse,  but  in  warmer 
climates  it  may  take  place  outdoors.  The 
inilial  cost  of  bed  preparation   may  be 


80  ] 


substantial,  but  when  one  considers  the 
potential  useful  period  in  terms  of 
seasons  or  years,  it  is  obviously  unwise 
to  economize  on  important  ingredients. 
Mix  B  may  be  satisfactory,  but  mix  C 
offers  greater  insurance  of  desirable 
physical  properties  for  optimum  growth. 
As  discussed  under  "Cans,"  it  may  be 
possible  to  substitute  other  organic  mate- 
rials for  part  of  the  mix,  to  provide  the 
best  possible  physical  conditions  at  re- 
duced cost.  In  commercial  establish- 
ments the  U.  C.-type  soil  mixes  (25  per 
cent  or  50  per  cent  peat)  have  given  good 
lateral  distribution  of  water  applied  to 
the  surface  by  the  porous-hose  or  drip 
system. 

Beds  might  be  prepared  by  using  mix 
C  and  fertilizer  II  (C)  or  V  (C)  if  they 
are  to  be  planted  at  once.  If  planting  is 
to  be  delayed,  fertilizer  I  (C)  or  IV  (C) 
should  be  used.  If  fertilizer  II  (C)  or 
V  (C)  is  used,  subsequent  application 
may  begin  4  to  6  weeks  after  planting. 
Regular  applications  of  dry  nitrogen 
sources  will  be  required  for  a  few 
months,  and  then  it  may  be  necessary  to 
use  one  of  the  mixed  dry  fertilizers  from 
time  to  time.  Because  of  the  length  of  the 
growing  period,  it  is  impossible  to  out- 
line a  reliable  long-term  procedure  here. 
Liquids  may  be  used  in  place  of  the  dry 
applications,  keeping  in  mind  that  nitro- 
gen will  be  required  at  first,  with  mixed 
materials  later.  If  fertilizer  I  (C)  or 
IV  (C)  is  used,  application  should  start 
1  to  2  weeks  after  planting,  with  subse- 
quent procedure  the  same  as  above. 

An  example  might  be  as  follows : 

Prepare  bed  with  mix  C  plus  fer- 
tilizer IV  (C). 

Apply  starter  solution  as  liquid 
L-2. 

Apply  liquid  L-2  every  third  irri- 
gation for  6  weeks,  then  shift  to 
liquid  L-7  every  other  irrigation. 

When    the    beds   are   renewed    or    re- 
planted more  peat  can  be  added. 

[ 


Planter  Boxes  and  Dish  Gardens 

Present-day  landscaping  makes  com- 
mon use  of  planter  boxes  both  indoors 
and  outside.  Standard  soil  mixes  of  the 
types  described  are  useful  for  a  wide 
range  of  plants.  For  large  boxes  with 
adequate  drainage  use  mix  B  or  C,  with 
the  appropriate  fertilizer  I,  II,  or  V,  fol- 
lowed by  the  same  feeding  program  as 
for  bench-  or  bed-grown  crops.  Dish 
gardens  or  beds  with  obstructed  drain- 
age may  use  mix  D  or  E  with  the  appro- 
priate fertilizer  I,  II,  or  V,  but  should 
receive  little  or  no  subsequent  fertilizing. 

Home-Yard  Planting 

Where  the  natural  soil  is  of  a  fine 
sandy  texture,  a  U.  C.-type  mix  may 
easily  be  prepared.  Where  the  natural 
soil  is  not  of  this  type,  an  expensive 
alternative  is  to  remove  and  replace  the 
existing  soil  with  the  more  desirable 
kind. 

When  setting  out  plants  grown  in  con- 
tainers of  fine  sand  and  peat  into  soil  of 
different  texture,  an  effort  should  be 
made  to  blend  the  soil  of  the  container 
with  the  existing  soil  so  that  a  transition 
zone  is  produced.  If  proper  planting  pro- 
cedures are  followed,  plants  raised  in  the 
fine  sand  and  peat  mixes  will  grow  very 
well  even  in  heavy  clay  soils.  The  exten- 
sive root  systems  produced  in  these 
mixes  favor  more  rapid  establishment 
when  transplanted. 

Frequently  it  is  necessary  in  landscape 
work  to  bring  in  top  soil.  The  U.  C.-type 
mixes  have  proved  successful,  simple  to 
handle,  and  relatively  inexpensive,  par- 
ticularly if  cheaper  organic  materials 
such  as  redwood  sawdust  or  shavings,  or 
rice  hulls  are  used  in  place  of  peat. 

For  Research 

It  is  common  practice  in  a  research 
glasshouse  to  steam  or  otherwise  treat 
soil  for  growing  test  plants,  to  protect 
them  from  soil-borne  diseases.  The  need 
for  healthy,  vigorous,  and  uniform  plants 

81] 


is  obvious  whatever  phase  of  botanical 
or  agricultural  science  is  under  investi- 
gation (p.  51).  The  importance  of  soil 
in  the  production  of  experimental  plants 
is  often  underrated,  and  wholly  unsuita- 
ble soil  types  used.  Poorly  grown  and  ex- 
ceedingly variable  plants  result. 

The  U.  C.-type  soil  mix  answers  the 
demands  for  growth  of  many  kinds  of 
experimental  plants.  In  studies  of  soil 
pathogens  it  may  be  used  (1)  immedi- 
ately after  steaming,  before  it  becomes 
extensively  recolonized,  or  (2)  after  the 
soil  has  again  developed  a  stable  flora 
from  air  contaminants,  contact,  or  in- 
oculation with  a  specific  flora.  For 
routine  tests  of  the  pathogenicity  of  or- 
ganisms on  underground  tissues,  the 
U.  C.-type  mix  in  such  a  biologically 
buffered  state  provides  conditions  similar 
to  those  in  a  natural  fertile  sandy  loam. 
When  tests  of  pathogenicity  and  the 
manifestation  of  symptoms  demand 
vigorously  growing  and  uniform  plants, 
a  U.  C.-type  soil  mix  is  generally  excel- 
lent as  a  growing  medium. 

There  are  also  examples  of  research 
for  which  the  desirable  attributes  of  the 
U.  C.-type  mix  disqualify  it.  Thus  it 
would  be  unsuitable  for  studies  on  re- 
sistance to  a  Phytophthora  root  rot 
which  normally  occurs  in  a  heavy,  poorly 
aerated  soil.  Wherever  the  qualities  of 
the  soil  itself  constitute  an  important 
factor  in  the  problem  under  study,  a 
natural  soil,  or  a  soil  mix  with  suitable 
properties,  should  be  given  preference. 

Apart  from  numerous  uses  in  the  study 
of  diseases  induced  by  fungi,  bacteria, 
and  nematodes,  the  U.  C.-type  mix  has 
proved  to  be  excellent  for  growing  plants 
for  virus  research.  In  this  field  the  re- 
quirements for  uniformity  are  often  as 
demanding  as  in  the  most  accurate 
physiological  studies  on  soil-grown 
plants. 

Turkish  tobacco  plants  have  been 
grown  l>\  J.  G.  Bald  and  P.  A.  Chandler3 
in  4-inch  pots  from  seed  to  a  height  of 
aboul    L5    inches,    bearing    15  to   18  ex- 


panded leaves,  within  a  period  of  7 
weeks  from  seeding.  The  average  rate 
of  increase  in  leaf  area  in  lots  of  600 
plants  has  reached  25  per  cent  per  day. 
From  the  emergence  of  the  first  true  leaf 
above  the  cotyledons  until  near  the  end 
of  this  period  the  leaf-area  growth  rate 
was  logarithmic,  provided  greenhouse 
conditions  remained  uniform.  Percentage 
increases  in  leaf  area  from  day  to  day 
were  the  same  whether  the  plants  were 
small  or  relatively  large.  For  much  of 
that  time  fresh  weight  of  the  tissues  was 
almost  linearly  related  to  leaf  area ;  later 
the  relation  was  more  complex,  but 
regular  and  predictable.  The  logarithmic 
growth  rate  was  reduced  by  the  crowd- 
ing of  roots  in  the  4-inch  pots  before  the 
physiological  changes  preceding  flower- 
ing could  take  effect.  By  growing  herba- 
ceous plants  in  larger  containers  of  a 
U.  C.-type  soil  mix  it  should  be  possible, 
for  a  particular  set  of  conditions,  to 
maintain  uniform  and  unrestricted 
growth  until  seeding.  It  was  possible  with 
this  soil  to  transplant  tobacco  seedlings 
in  the  cotyledon  stage  without  loss,  or 
even  a  noticeable  check  to  growth. 

The  physical  characteristics  are  so 
good  that  the  first  roots  of  the  tiniest 
seedling  or  of  the  largest  plant  pass 
directly  through  the  soil  mass  without 
being  diverted  to  the  edge  of  the  con- 
tainer (fig.  61) . 

In  addition  to  rapid  growth,  experi- 
mental plants  in  a  U.  C.-type  mix,  if 
correctly  selected  and  handled,  exhibit 
remarkably  uniform  size  and  habit. 
There  are  several  factors  unrelated  to  the 
soil  mix  which  affect  uniformity  of  plant 
growth.  Among  them  are:  (1)  heritable 
variability  between  seedlings,  even 
within  a  horticultural  variety;  (2) 
rapidity  of  seed  germination;  (3)  dif- 
ferences in  extent  of  roots,  caused  in  part 
by  development  of  recolonized  non- 
pathogenic organisms  in  any  treated 
soil;    (4)   other  random  sources  of  vari- 

1  Department  of  Plant  Pathology,  University 
of  California,  Los  Angeles. 


[82] 


Fig.  61.  Root  development  of  Croft  lily  in  U.  C.  mix  C  (50  per  cent  peat).  Plant  at  left  in  the 
soil  ball,  at  right  washed  free  of  soil.  Note  the  size  of  the  root  system  and  its  development 
throughout  the  ball. 


ability.  A  group  of  plants  grown  in  a 
U.  C.-type  soil  mix  may  superficially 
appear  more  variable  than  those  grown 
in  other  soil  because  the  more  rapid 
healthier  growth  accentuates  the  inherent 
differences.  For  example,  a  plant  arising 
from  a  seed  that  germinated  3  days  later 
than  its  neighbors  will  appear  relatively 
smaller  in  a  rapidly  growing  series  than 
it  will  in  one  of  poor  growth.  Plants 
grown  under  optimum  conditions  in  a 
soil  mix  of  the  U.  C.  type  may  be  sorted, 
accurately  matched,  and  practically  all 
undesirable  plants  eliminated,  with  the 
assurance  that  the  maximum  variability 
is    revealed.    Such    selected    plants    are 


often  comparable,  leaf  by  leaf,  from  the 
cotyledons  to  the  growing  tip,  and  they 
will  remain  so  through  the  period  of  the 
experiment.  On  the  other  hand,  poorly 
grown  plants  cannot  be  matched  in  this 
way.  Their  apparent  similarity  may  mask 
sources  of  variation  that  eliminate  all 
chance  of  obtaining  accurate  information 
from  experiments  in  which  they  are  used. 
The  U.  C.  mixes  will  prove  very  useful 
in  the  research  greenhouse  wherever 
the  production  of  uniform,  well-grown 
plants  is  desired  for  experimental  studies. 
The  uniform  growth  rate  may  be  main- 
tained during  the  experiment  if  other 
conditions  are  favorable. 


PRACTICAL  CONSIDERATIONS 


Preparation  of  the  Mixes 

The  mixing  of  the  fine  sand,  peat  or 
other  organic  material,  and  fertilizer 
components   can    be   very    simple.    Peat 


should  be  wetted  before  being  mixed, 
preferably  1  or  2  days  before  use.  The 
fine  sand,  peat  or  other  organic  material, 
and  the  mixed  fertilizers  should  be  in 


[83] 


piles  convenient  to  the  mixing  operation. 
Complete  mixing  is  essential. 

If  mixing  is  done  by  hand,  the  proper 
amounts  of  the  various  ingredients 
should  be  placed  in  a  low,  level  pile  with 
the  fertilizer  components  broadcast 
evenly  over  the  surface.  This  should  be 
turned  with  a  shovel,  progressively  work- 
ing through  the  mass  from  one  side  and 
forming  a  second  pile  as  the  shovels  of 
soil  are  turned  over.  This  second  pile  is 
then  turned  back  again,  and  the  process 
repeated  until  blending  is  complete. 

If  mixing  is  by  machine,  it  is  generally 
done  with  a  concrete  mixer  (Sec.  17)  or 
a  ribbon  mixer.  In  the  smaller  nursery  a 
small  cement  mixer  may  be  used  and 
ingredients  added  by  hand.  In  larger 
operations,  the  major  ingredients  (fine 
sand,  peat)  are  generally  placed  in  the 
mixer  with  a  skip  loader  and  the  fertilizer 
components  added  by  hand. 

Moist,  but  not  excessively  wet,  base 
ingredients  are  essential  to  uniform  mix- 
ing and  to  reliability  of  subsequent  treat- 
ment. 

Watering  Practices 

Since  fine  sands  do  not  have  cementing 
properties  (Sec.  6),  the  particles  can  be 
readily  dislodged.  If  heavy  streams  of 
water  are  applied  to  fine  sand  and  or- 
ganic mixes,  the  surface  is  churned  up, 
and  the  sand  settles  first.  The  organic 
matter  therefore  collects  on  the  surface. 
Where  mulches  are  undesirable,  it  is 
necessary  to  use  water  breakers  during 
irrigation  to  avoid  this  effect  on  U.  C- 
type  mixes. 

Some  adjustment  in  watering  prac- 
tices is  generally  desirable  when  the 
grower  first  uses  a  U.  C.-type  mix  if  he 
has  been  accustomed  to  a  clay  soil. 

Dump  Soil 

In  any  necessary  operation  a  certain 
amount  of  used  soil  is  apt  to  accumulate. 
This  is  probably  more  true  of  the  bed- 
ding-plant  operation   than  of  any  other. 


J.  L.  Mather"  found  that  in  15  repre- 
sentative California  bedding-plant  nur- 
series an  average  of  16  per  cent  of  the 
flats  were  dumped.  The  problem  of  dis- 
posal of  this  material  sometimes  assumes 
major  proportions.  Since  the  physical 
texture  and  structure  of  used  U.  C.-type 
soil  will  be  quite  acceptable,  the  problem 
is  centered  on  its  chemical  properties.  If 
it  is  to  be  re-used,  it  is  necessary  to  know 
whether  to  add  fertilizer,  and  if  so,  how 
much. 

For  obvious  reasons  no  standard  pro- 
cedure can  be  proposed  which  would 
take  into  account  all  the  possible  varia- 
tions which  will  exist.  Several  sugges- 
tions, however,  may  be  made: 

1.  Sell  the  material  as  top  soil  for 
landscaping  or  similar  use  where 
physical,  rather  than  chemical, 
properties  are  of  prime  importance. 

2.  Check  the  salinity  of  the  soil  by 
the  saturation-extract  conductance 
method  (Sec.  4)  and,  if  it  is  satis- 
factorily low,  use  a  portion  of  this 
soil  as  a  substitute  for  the  fine  sand 
in  the  standard  mix. 

3.  Determine  by  adequately  complete 
analysis  the  exact  nutrient  status  of 
the  used  soil,  and  calculate  exactly 
what  to  add  in  order  to  bring  it  up 
to  standard. 

Cost  of  the  Soil  Mixes 

Cost  of  a  soil  mix  will  be  determined 
by  the  cost  of  the  soil,  transportation  to 
the  nursery,  organic  ingredients,  ferti- 
lizers, and  labor  in  mixing.  A  major  dif- 
ference in  cost  between  the  older  type  of 
composted  mixes  and  those  of  the  U.  C. 
type  is  that  of  labor.  With  a  compost  the 
materials  are  handled  at  least  2  or  3 
times  before  treatment,  whereas  mixes 
of  the  U.  C.  type  require  only  one  opera- 
tion. 

There  is  also  the  lower  cost  of  the 
materials    themselves.    Fine    sands    will 

8  Manager  of  the  former  Bedding  Plant  Ad- 
visory Hoard,  Bureau  of  Marketing,  California 
Stale  Department  of  Agriculture. 


[84] 


usually  be  cheaper  than  top  soil  because 
of  the  more  efficient  machinery  that  can 
be  used  in  digging  the  material,  often  to 
a  considerable  depth.  Top  soil  often  is 
more  costly  because  only  the  surface 
layer  of  limited  areas  is  removed,  using 
less  efficient  machinery.  In  many  areas 
the  price  for  top  soil  is  greater  than  for 
fine  sand  because  of  competition  for  it. 
Soil-conservation  practices  also  often 
forbid  the  removal  of  top  soil.  Some  of 
the  fine  sands  that  are  suitable  for  use  in 
U.  C.-type  mixes  are  actually  waste 
products  from  the  screening  and  washing 
of  building  materials. 

Probably  the  main  factor  in  any  soil 
cost  will  be  the  transportation  charge. 
Every  nurseryman  should  try  to  locate  a 
source  as  close  to  his  nursery  as  possible. 

Assuming  an  average  33  per  cent 
shrinkage  of  leaf  mold,  manure,  or  other 
compost  materials,  the  cost  per  useful 
unit  volume  should  be  increased  by  one 
half  over  the  purchase  price.  One  cubic 
foot  of  baled  peat,  on  the  other  hand, 


yields    1.5    to    1.6    cubic    feet    of    loose 
material. 

Besides  the  lower  labor  requirement 
mentioned  above,  comparative  cost  of 
ingredients,  mixes,  and  composts  shows 
that  the  U.  C.-type  mix  is  less  expensive 
than  other  common  soil  preparations. 
Computed  at  1955  wholesale  prices,  de- 
livered to  a  near-by  nursery  in  the  Los 
Angeles  area,  the  cost  per  cubic  yard  of 
some  ingredients  is  as  follows: 

Fine  sand   S  2.00 

Top  soil $  3.00 

Peat  moss,  loose $  6.75 

($4.50  per  12  cu.  ft.  bale) 

Sawdust    $  1.00 

Leaf  mold $13.50 

($9.00  per  cu.  yd.  delivered;  33  per  cent 
shrinkage  sustained) 

Steer  manure    $  9.00 

($6.00  per  cu.  yd.  delivered;  33  per  cent 
shrinkage  sustained) 
Using   these   materials,    the    cost    per 
cubic  yard  of  various  nursery  soil  mix- 
tures   may    be    computed    as    shown    in 
table  9. 


Table  9.  Comparative  Cost  of  Common  Soil  Mixes 


Soil  mix 


STANDARD  U.  C.  MIXES 

A  (100%  fine  sand) 

B  (75%  fine  sand,  25%  peat) 

C  (50%  fine  sand,  50%  peat) 

D  (25%  fine  sand,  75%  peat) 

E  (100%  peat) 

U.  C.  MIXES  USING  SAWDUST 
B  (75%  fine  sand,  25%  sawdust) 
C  (50%  fine  sand,  50%  sawdust) 
D  (25%  fine  sand,  75%  sawdust) 
E  (100%  sawdust) 

COMPOST 

75%  top  soil,  25%  leaf  mold.  . . . 
50%  top  soil,  25%  leaf  mold, 

25%  manure 

50%  top  soil,  50%  leaf  mold 

100%  leaf  mold 


Cost  of  ingredients  per  cubic  yard  of  mix 


Soil 


$2.00 
1.50 
1.00 
0.50 


1.50 
1.00 
0.50 


2.25 

1.50 
$1.50 


Peat 


$1.69 
3.37 
5.06 

$6.75 


Saw- 
dust 


$0.25 
0.50 
0.75 

$1.00 


Leaf 
mold 


$3.37 

3.37 

6.75 

$13.50 


Manure 


$2.25 


Total 


$2.00 
3.19 
4.37 
5.56 
6.75 

1.75 
1.50 
1.25 
1.00 

5.62 

7.12 

8.25 

$13.50 


[85] 


SECTION 


Components  and 
Development  of  Mixes 


O.  A.  Matkin 
Philip  A.  Chandler 
Kenneth  F.  Baker 


Functions  of  the  soil 
Disadvantages  of  multiple  soil  mixes 
Attempts  to  improve  nursery  soil  mixes 
Soil  toxicity  in  relation  to  treatments 
Criteria  for  physical  ingredients  of  soil  mixes 
Selecting  ingredients  for  U.C.  mixes 


o 


ne  OF  the  commonest  erroneous 
ideas  in  nursery  practice  is  that  a  special 
soil,  resembling  as  closely  as  possible  the 
soil  of  its  native  habitat,  is  required  for 
each  type  of  plant.  This  involves  the  fal- 
lacious assumption  that  distribution  of 
wild  plants  is  determined  by  soil  type, 
whereas  actually  the  temperature,  rain- 
fall, day  length,  light  intensity,  soil 
salinity,  the  point  of  origin,  as  well  as 
other  factors,  are  at  least  as  important  in 
determining  where  plants  grow.  It  may 
actually  be  misleading  to  assume  that  the 
best  soil  for  a  plant  is  that  of  its  native 
habitat,  since  the  plant  may  have  had  to 
"tolerate"  that  soil  because  another 
factor,  such  as  frost,  may  have  limited  it 
to   that  particular  area.  Most   plants  of 


necessity  must  have  a  wide  tolerance  to 
soil  types  in  order  to  survive. 

The  soil  used  may  simply  be  a  matter 
of  tradition.  Some  growers  plant  verbena 
in  straight  leaf  mold  although  it  is  not  a 
native  of  dense  woodlands,  and  lilies  in 
black  adobe  although  they  require  good 
drainage.  The  surprisingly  good  results 
sometimes  obtained  in  such  media  testify 
to  the  tolerance  of  plants  in  this  regard. 

The  John  Innes  Horticultural  Institu- 
tion (Bayfordbury,  Hertfordshire,  Eng- 
land) demonstrated  in  1934-1939  that 
many  kinds  of  plants  could  be  grown  in 
a  single  soil  mix,  or  in  slight  modifica- 
tions of  it.  As  this  concept  has  been 
recognized  by  growers,  there  has  been  a 
trend  away  from  specialized  mixes  for 
each  type  of  plant. 


FUNCTIONS  OF  THE  SOIL 


Any  good  growing  medium  must  pro- 
vide for  the  basic  requirements  of  the 
plants  in  it.  Since  all  green  plants  have 


the  same  basic  requirements,  the  prob- 
lem is  simplified.  The  growing  medium 
supplies  only  the  following  functions. 


[86] 


Support 

Most  crops  require  some  means  of 
physical  support.  Unless  artificially  pro- 
vided, this  is  a  function  of  the  growing 
medium;  support  is  not  a  factor  of  major 
concern  unless  the  plant  is  large  and  the 
growing  medium  of  very  light-weight 
material  such  as  peat  moss.  In  nursery 
growing  it  is  common  to  use  stakes  and 
ties  of  various  types  to  support  plants  in 
small  containers. 

Moisture 

The  living  plant  is  largely  composed 
of  water,  which  must  be  obtained  from 
the  soil  in  which  it  grows.  A  good  grow- 
ing medium  should  have  a  reasonable 
ability  to  hold  moisture  in  sufficient 
supply  for  plant  requirements  between 
irrigations.  Water  is  more  often  limiting 
to  plant  growth  than  such  items  as  ferti- 
lizer, salinity,  or  alkalinity,  which  are  so 
often  blamed. 

High  salinity  (Sec.  4)  may  virtually 
make  soil  water  unavailable  to  the  plant 
because  dissolved  salts  increase  the  os- 
motic pressure  in  the  soil  solution.  If  the 
concentrations  outside  the  root  approach 
those  within  it,  owing  to  dissolved  salts, 
water  movement  into  the  plant  is  re- 
stricted. 

Since  containers  have  limited  depth, 
a  boundary  exists  at  the  bottom  in  con- 
trast to  a  continuous  soil  column  in  the 
field.  This  boundary  constitutes  a  restric- 
tion to  free  drainage  (Baver,  1956; 
Huberty,  1945).  Thus,  soil  in  a  con- 
tainer will  retain  more  moisture  avail- 
able to  plants  after  an  irrigation  than  it 
would  in  the  field. 

Large  quantities  of  water  are  lost  by 
the  plant  through  transpiration;  when 
the  plant  wilts,  this  indicates  that  loss  is 
greater  than  the  supply  from  the  roots. 
Although  this  is  the  major  plant  use  of 
water,  it  is  bv  no  means  the  only  im- 
portant one.  Water  is  the  solvent  in  which 
minerals  are  taken  into  and  transported 
through    the    plant.    The    two    elements 


comprising  water,  hydrogen  and  oxygen, 
play  individually  important  roles  in  plant 
metabolism.  All  of  the  organic  materials 
of  plants  contain  large  quantities  of  each. 
The  fact  that  plants  can  be  grown  in 
water  (culture-solution  growing)  in- 
dicates that  there  is  no  such  thing  as  ex- 
cessive water  where  the  other  basic  re- 
quirements are  satisfactorily  met.  On  the 
other  hand,  plant  growth  unquestionably 
can  be  restricted  by  conditions  which 
subject  the  plant  to  increasingly  deficient 
moisture.  Frequently  this  point  is  over- 
looked by  the  grower  unless  he  happens 
to  have  a  comparison  available.  The  ac- 
cumulative stunting  effect  is  shown  dia- 
grammatically  in  figure  62. 

Aeration 

The  roots  of  a  plant  obtain  the  raw 
materials,  water  and  mineral  nutrients, 
which  are  carried  upward  through  the 
stem  to  the  leaves.  The  tops  act  as  fac- 
tories, synthesizing  the  compounds  re- 
quired for  growth  and  reproduction 
from  these  materials  and  carbon  dioxide 
from  the  air.  For  roots  to  function 
normally  they  must  be  supplied  with  a 
source  of  energy  and  an  environment 
favorable  for  utilizing  it.  The  top  of  the 
plant  provides  the  sugars  and  other 
carbohydrates,  which  are  transported 
through  the  stem  down  to  the  roots, 
where,  through  respiration,  they  supply 
the  energy  necessary  for  root  function. 

Respiration,  as  in  the  case  of  animals, 
requires  oxygen  and  produces  carbon 
dioxide  and  water.  Oxygen  is  also  re- 
quired for  respiration  in  other  parts  of 
the  plant,  but  the  supply  there  is  nearly 
always  adequate.  Because  of  the  tiny 
pore  spaces  in  soil  through  which  the 
gases  move,  aeration  (oxygen  supply  and 
carbon  dioxide  removal)  of  the  roots 
can  readily  become  limiting.  A  good  soil 
mix  must  insure  the  best  possible  aera- 
tion consistent  with  other  requirements. 
The  additional  moisture  retained  by  soil 
in  a  container  reduces  the  air  space.  It  is. 
therefore,  important  that  container  soils 


[87] 


Plant 
Growth 


Available 

Soil 
Moisture 

Unavailable 

Soil 

Moisture 


CONSTANTLY  AVAILABLE 
WATER  SUPPLY 


INTERMITTENTLY  AVAILABLE 
WATER  SUPPLY 


Time 


Time 


Fig.  62.  Diagram  of  plant  growth  in  relation  to  moisture  availability.  The  plant  constantly  sup- 
plied with  water  grows  continuously.  The  plant  exposed  to  occasional  water  deficit  grows  inter- 
mittently, and  is  smaller. 


Fig.  63.  Diagram  of  plant  growth  in  relation  to  mineral  nutrient  supply.  The  plant  uniformly 
supplied  with  fertilizer  grows  continuously  and  is  larger  than  the  one  intermittently  supplied. 
An  excessive  application  of  fertilizer  (at  right)  killed  the  plant. 


Plant 
Growth 


Tissue 
damage 

Slight 
excess 

Favorable 
supply 

Deficiency 


CONSTANTLY  FAVORABLE 
MINERAL  NUTRIENT  SUPPLY 


VARYING 
MINERAL  NUTRIENT  SUPPLY 


Death 

f 

Fertilizer  Application 

Fertilizer  Application 

s  \ 

p^v 

\N. 

Time 


Time 


[  88  | 


have  a  maximum  porosity.  It  is  pri- 
marily by  diffusion  that  gases  move  into 
and  out  of  a  soil,  though  applications  of 
water  may  also  be  effective  in  displacing 
soil  air,  particularly  in  containers  (Sec. 
9) .  If  the  soil  pore  spaces  are  very  small, 
water  will  fill  them  and  reduce  aeration 
until  the  water  content  has  been  lowered 
by  evaporation  or  transpiration. 

Of  additional  importance  is  the  fact 
that  a  soil  through  which  air  does  not 
diffuse  readily  will  also  be  difficult  to 
treat  efficiently  by  fumigation  or  steam 
(sees.  8,  9,  and  11) . 

Mineral  nutrients 

At  the  present  time  most  green  plants 
are  known  to  require  at  least  twelve 
chemical  elements  (nitrogen,  phospho- 
rus, potassium,  calcium,  magnesium, 
sulfur,  iron,  zinc,  manganese,  copper, 
boron,  and  molybdenum)  that  are  ob- 
tained from  the  growing  medium  by  the 


roots.  Foliar  feeding  may  be  used  to  sup- 
plement root  absorption.  A  fertile  soil  is 
one  in  which  all  of  these  elements  are 
present  in  adequate  but  not  excessive 
quantity.  A  good  soil  mix  must  therefore 
contain  them,  or  the  growing  procedure 
must  provide  for  their  supply  during 
plant  growth. 

This  function  is  made  possible  in  part 
through  the  breakdown  of  organic  mat- 
ter, native  mineral  soils,  and  fertilizers 
in  the  complex  activities  of  soil  microor- 
ganisms, as  well  as  fixing  atmospheric- 
nitrogen  to  make  it  available  to  the  plant 
(Sec.  14).  To  this  extent  they  are  prop- 
erly considered  as  a  necessary  part  of 
the  soil  environment  of  the  plant. 

As  with  moisture,  it  is  important  that 
the  supply  of  these  minerals  be  continu- 
ous rather  than  intermittent  (fig.  63). 
The  greatest  problem  occurs  in  main- 
taining proper  nitrogen  supply  (Sec.  7). 


DISADVANTAGES  OF  MULTIPLE  SOIL  MIXES 


It  is  still  the  practice  in  some  Califor- 
nia nurseries  to  have  a  separate  bin  or 
compost  pile  of  a  special  soil  mix  for 
nearly  every  crop  grown.  This  is  a  costly 
procedure  beset  with  several  serious  dis- 
advantages. 

Labor  requirement 

Preparation  of  many  small  lots  of  soil 
costs  vastly  more  than  does  preparation 
of  a  single  large  batch,  and  makes  un- 
economic the  mechanization  of  handling. 
If  compost  piles  or  bins  are  maintained 
for  each,  the  labor  requirement  becomes 
very  large. 

Space  utilized 

Land  area  is  required  for  the  piles  of 
raw  materials  or  compost,  mixing  areas, 
and  storage  bins.  It  is  not  uncommon  for 
large  nurseries  in  southern  California  to 
use  1  to  2  acres  for  these  purposes.  Be- 
cause   of   the   real-estate   pressures   pre- 


viously mentioned  (Sec.  2),  and  the  in- 
creasing tax  rates,  land  area  must  now 
be  used  with  greatest  efficiency. 

Variability  of  composts 

Composts  containing  leaf  mold,  animal 
manure,  or  turf  will  be  highly  variable 
in  composition  because  these  materials 
are  themselves  far  from  uniform.  Fur- 
thermore, if  these  ingredients  are  com- 
posted, the  degree  of  decomposition  will 
not  be  uniform  in  all  lots  at  different 
seasons,  and  the  mixture  thus  would  be 
even  more  variable  than  before.  With 
these  inherently  variable  mixtures,  the 
plant  response  often  becomes  so  un- 
predictable as  to  prevent  scheduled  pro- 
duction. 

Shrinkage  in  composting 

Manure,  used  to  supply  organic  matter 
and  nutrients,  is  not  a  good  source  of 
either  for  nurseries.   In  California  it  is 


[89] 


likely  to  be  more  dehydrated  than  de- 
composed, and  the  buyer  accordingly  as- 
sumes the  shrinkage.  "Leaf  mold"  in 
California  usually  means  partly  decayed 
leaves,  subject  to  a  considerable  loss  in 
composting.  Because  peat  is  largely  de- 
composed before  it  is  dug,  composting  is 
unnecessary  and  shrinkage  during  use  is 
comparatively  minor. 

Odor   and   flies   during 
composting 

In  residential  areas  unpleasant  odors 
and  flies  are  likely  to  bring  zoning  re- 
strictions. Compost  piles  are  already 
being  discontinued  for  this  reason  in 
some  areas. 

Scarcity  of  composting 
materials 

Because  of  the  scarcity  of  deciduous 
forests  in  California,  the  semiarid  cli- 
mate, and  destruction  from  fires,  leaf- 
mold  deposits  are  rare  and  generally 
protected  by  law.  Animal  manure  may 
become  scarce,  and  the  nurseryman  must 
compete  with  mushroom  growers  and  the 
package-manure  trade  to  get  it.  For  these 
reasons  many  nurserymen  have  prac- 
tically ceased  using  these  two  materials. 
In  England  and  the  humid  parts  of  this 
country,  sod  from  turf  or  meadowland  is 
used  in  compost  piles.  In  California,  turf 
can  rarely  be  used  because  there  are  few 
natural  meadows,  and  land  and  water 
are  too  valuable  to  be  used  only  for  a 
turf  crop. 

Salinity  problem 

This  serious  problem,  discussed  in 
Section  4,  is  important  in  the  selection  of 
composting  materials.  During  the  decom- 


position of  leaf  mold  in  place,  in  the 
compost  pile,  or  in  the  container,  the 
mineral  content  is  made  soluble,  which 
increases  salinity.  Consequently,  Califor- 
nia "leaf  mold"  is  commonly  high  in 
soluble  salts,  whereas  in  areas  of  high 
rainfall  these  have  been  removed  by 
leaching.  In  one  instance,  leaf  mold  that 
had  been  used  for  growing  cattleya  or- 
chids in  benches,  was  sold  to  nurserymen 
for  use  in  bedding  plants.  This  decom- 
posed material  had  excellent  physical 
properties,  but  because  it  had  been 
watered  lightly  without  leaching,  the 
salts  from  the  water  and  from  fertilizers 
had  accumulated  to  an  extremely  high 
level.  When  it  was  used  in  nursery  soils, 
seedlings  were  quickly  and  severely  in- 
jured by  the  soluble  salts.  Manures,  by 
their  very  nature,  are  always  saline. 

Post-treatment  toxicity 

This  important  disadvantage  of  con- 
ventional soil  mixes  is  discussed  below. 

Comparison   with 
U.  C.-type   mixes 

By  comparison  with  multiple  soil 
mixes,  the  use  of  a  single  one  of  the  U.  C. 
type  (Sec.  5)  presents  definite  advan- 
tages: it  requires  less  labor  and  can  be 
more  economically  mechanized  in  the 
handling  operation;  less  storage  space  is 
needed  since  compost  piles  are  elimi- 
nated; greater  uniformity  of  mixture 
and  predictability  of  plant  growth  will 
result;  loss  by  shrinkage  and  leaching 
during  composting  is  reduced;  scarce 
materials,  such  as  manure  and  leaf  mold, 
may  be  eliminated  by  using  readily 
available  peat  instead;  some  of  the 
sources  of  soluble  salts  may  be  avoided. 


ATTEMPTS  TO  IMPROVE  NURSERY  SOIL  MIXES 

Progress   in   the  art  of  compounding  artificial    conditions    imposed    by    man. 

nursery  soil   mixtures  is  but  an  expres-  Empirical   additions    of   different   mate- 

sion     of     the     developing     fundamental  rials,    or    the    formulation    of    various 

philosophies  of  plant  culture  under  the  mixes,  are  of  less  permanent  importance 

[  90  1 


to  nurserymen  than  is  the  evolution  of 
the  ideas  behind  them.  Viewed  from  this 
angle  there  has  been  slow  but  steady 
progress  in  the  subject. 

Man's  earliest  cultivation  of  plants 
was  undoubtedly  in  some  sort  of  field 
plots,  and  he  found  at  a  fairly  early 
period  that  growth  was  enhanced  by  ap- 
plication of  some  type  of  fertilizer.  Per- 
haps he  discovered  that  plants  would 
grow  in  containers  when  he  was  faced 
with  the  necessity  of  moving  some  of 
them  to  a  new  area  during  migrations.  It 
was  then  but  a  step  to  find  that  they 
could  be  grown  in  that  way  very  suc- 
cessfully if  fertilized  and  properly  han- 
dled. It  is  known  that  trees  were  grown 
in  large  "boxes"  or  "pots"  cut  in  rock 
and  filled  with  special  soil  in  Egypt  about 
4,000  years  ago.  Frankincense  trees  were 
brought  from  Punt  (Somali  Coast)  to 
Egypt  in  containers  to  be  grown  in  gar- 
dens about  3,500  years  ago;  that  this  was 
carefully  recorded  as  an  outstanding 
achievement  suggests  that  it  was  one  of 
the  earliest  instances  of  plant  nursery 
operations  (fig.  64 J.1  When  man  began 
growing  plants  for  ornamental  use  it  was 
natural  that  some  should  be  grown  in 
containers  to  be  taken  into  his  home  or 
gardens. 

When  a  practice  is  slowly  developed, 
there  usually  is  little  critical  examina- 
tion of  the  methods  employed.  Growers 
of  plants  in  containers  adopted  a  very 
complex  series  of  soil  mixtures  for  dif- 
ferent crops.  Although  many  growers 
thought  they  had  independently  de- 
veloped the  ideal  soil  for  a  given  crop, 
there  was  no  consistency  between  them, 
as  there  should  have  been  if  the  con- 
clusions were  valid.  Sometimes  this  mix 
was  based  on  the  soil  that  had  been  used 
for   an    especially    successful   crop,    the 

1  The  Temples  of  Neb-hepet  Re'  Mentu-hotpe 
(2061-2010  B.C.)  (Winlock,  1942),  and  Hat- 
shepsGt  (1520-1479  B.C.),  Deir  el-Bahri,  near 
Thebes  (Naville,  1913).  The  Metropolitan  Mu- 
seum of  Art,  New  York,  directed  the  writers  to 
these  examples. 


possibility  that  some  other  factor  may 
have  been  largely  responsible  for  the 
superior  results  being  ignored.  In  other 
cases  this  was  based  on  the  fallacious  as- 
sumption, already  discussed,  that  the 
only  proper  soil  for  a  plant  is  one  similar 
to  that  of  its  native  habitat. 

The  J.  I.  Composts 

Tests  at  the  John  Innes  Horticultural 
Institution  in  England  demonstrated  that 
a  single  soil  mixture  could,  with  minor 
modifications,  be  used  for  growing  a 
wide  range  of  plants.  The  usual  battery 
of  special  mixes  was,  therefore,  unneces- 
sary. The  importance  of  this  finding  has 
slowly  been  appreciated  by  growers  in 
England,  Europe,  and  this  country.  This 
development  constitutes  one  of  the  im- 
portant conceptual  landmarks  in  the  sub- 
ject of  nursery  soils.  For  the  English 
grower  this  has  been  crystallized  in  the 
J.  I.  composts,  as  follows: 

Seed  compost 
7  parts  composted  medium  loam,  by  volume 
3^j  parts  peat,  by  volume 
3^/2  parts  coarse  sand,  by  volume 
To  each  cubic  yard  of  the  above  is  added, 
with  thorough  mixing 

2  pounds  of  superphosphate  (18  per  cent 
phosphoric  acid) 

1  pound  of  chalk  (calcium  carbonate) 

Potting  compost 
7  parts  composted  medium  Loam,  by  volume 

3  parts  peat,  by  volume 

2  parts  coarse  sand,  by  volume 

To  each  cubic  yard  of  the  above  is  added, 
with  thorough  mixing 

2  pounds  of  hoof  and  horn  meal  (13  per  cent 
nitrogen) 

2  pounds  of  superphosphate  (18  per  cent 
phosphoric  acid) 

1  pound  of  sulfate  of  potash  (48  per  cent 
potash) 

1  pound  of  chalk  (calcium  carbonate) 

The  medium  loam  consists  of  the  com- 
posted residue  of  a  4-  to  5-inch  layer  of 
turf  removed  from  pastures  or  meadows; 
it  contains  2  to  7  per  cent  humus  and 
only  enough  clay  "to  be  slightly  greasy 
when  smeared".  Sandy,  heavy,  or  cal- 
careous soils  are  said  to  be  unsatisfac- 
tory. 


[91] 


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r 


mm 


-ifif] 


w,\\v\\ 


yyiStl  I 


/  ///V^7/  /  /  /AC////  /  /  /  / 


'2Jjlj.ll  )j)iU,  J/  /  /  /  /  /jhA^ufij  /  /  /  /  /  /  /  /  /  /  //_.v '/'// //////////'/) //)//////////// a?)  i 


In  order  to  avoid  toxic  residue  from 
this  mix  it  was  found  necessary  to  steam 
the  composted  loam  before  adding  the 
peat,  then  to  mix  them  and  add  the  lime 
and  fertilizers. 

The  following  disadvantages  are  in- 
herent in  the  J.  I.  composts:  (1)  some 
variability  necessarily  results  from  the 
use  of  composted  nonuniform  turf  soil; 
(2)  the  necessary  composting  of  the  turf 
loam  before  using  in  the  mix  takes  both 
time  and  space;  (3)  there  is  a  high  labor 
requirement  in  handling  the  composting 
operation;  (4)  meadowland  turf  is  not 
commonly  available  in  most  areas  in  this 
country;  (5)  they  contain  coarse  sand, 
which  unnecessarily  increases  the 
weight;  (6)  a  toxic  residue  is  apparently 
produced  if  the  soil  is  steamed  after  mix- 
ing. If  toxicity  is  prevented  by  aging 
after  steaming,  the  recontamination 
problem  is  increased  and  a  storage  prob- 
lem is  created;  if  it  is  prevented  by 
steaming  the  components  separately  and 
then  mixing,  the  recontamination  prob- 
lem is  still  increased. 

Evolving  the  U.  C.-Type  Mixes 

Work  was  begun  in  the  Department  of 
Plant  Pathology,  University  of  Califor- 
nia, Los  Angeles,  in  1941  to  find  a  better 
soil  for  growing  plants  in  containers 
than  that  then  available.  Because  of  the 
scarcity  of  turf  for  composting,  the  toxic 
residue  after  steaming,  and  the  salinity 
problem,  it  was  not  possible  to  use  the 
J.  I.  compost  system.  A  substitute  was 
therefore  sought.  At  first  a  fine  sandy 
loam  was  selected,  and  to  this  was  added 
leaf  mold  and  horse  manure  in  the  ratio 
of  10:2:2.  Because  of  the  variability  of 
these  organic  materials,  and  the  potential 
danger  of  excess  salts  in  them,  Canadian 


peat  was  soon  substituted  for  them  in  the 
ratio  of  3  parts  of  peat  to  7  parts  of 
sandy  loam.  Mineral  fertilizers  were 
added  before  steaming.  No  toxic  effect 
was  observed  in  any  of  a  wide  range  of 
plants  grown  in  this  mix,  even  when 
planted  immediately  after  steaming. 
Work  to  this  stage  of  development  was 
reported  in  1948  (Baker,  1948).  This 
mix  was  used  for  several  years. 

Further  developments  of  the  mix  were 
undertaken  by  the  Department  of  Plant 
Pathology  in  1949  when  Philip  A. 
Chandler,  who  had  been  at  the  John 
Innes  Horticultural  Institution  in  1935- 
1937,  joined  the  group.  0.  A.  Matkin  co- 
operated in  the  work  after  1950,  particu- 
larly on  physical  and  chemical  aspects  of 
the  soils.  Five  representative  formula- 
tions of  two  ingredients,  fine  sand  and 
peat,  with  a  number  of  variations  of 
fertilizer  additions  are  now  suggested 
(Sec.  5).  Collectively  these  have  been 
named  the  U.  C.  soil  mix,  and  sometimes 
erroneously  called  "the  UCLA  blend," 
"Cal-Mix,"  "light  soil  mix,"  and  "Calsoil 
Mix." 

The  concepts  behind  this  system,  how- 
ever, are  of  greater  permanent  value  to 
the  industry  than  the  formulas.  Toxicity 
after  steaming  has  been  eliminated, 
probably  through  the  use  of  fine  sand 
without  appreciable  clay  or  organic- 
matter  fraction,  and  of  peat  free  of 
readily  decomposable  organic  matter, 
and  through  maintenance  of  low  con- 
centrations of  soluble  salts  by  using  low- 
conductance  ingredients  and  fertilizers 
not  readily  decomposed  by  steaming. 
The  mixes  are  reproducible  because  they 
use  only  ingredients  that  are  readily 
available  in  uniform  quality.  Compost- 
ing  is   eliminated   because   the   organic 


Fig.  64.  Culture  of  plants  in  containers  was  perhaps  first  practiced  by  the  Egyptians  3,500- 
4,000  years  ago.  Top,  frankincense  trees  growing  in  pots.  Middle  and  bottom,  frankincense  trees 
in  containers  being  introduced  to  Egypt  from  the  Somali  Coast.  This  is  one  of  the  early  recorded 
instances  of  plant  introduction.  Recorded  in  the  Temple  of  Hatshepsut,  Deir  el-Bahri,  near  Thebes. 
(From  Naville,  1913.) 


[93] 


matter  used  is  already  largely  broken 
down,  and  the  fertilizer  is  uniformly  dis- 
tributed through  the  mass  by  mechanical 
means  rather  than  "weathering".  Plant 
growth  in  the  mixes  is  uniform  in  size 
and  time.  Many  kinds  of  plants  obtain 
the  necessary  nutrients  in  uniform  sup- 
ply from  these  mixes  for  several  weeks 
before  additional  fertilizer  is  needed. 
When  organic  nitrogen  is  used,  plants 
are  supplied  a  minimum  level  of  nitro- 
gen below  which  the  supply  does  not  fall. 
The  organic  nitrogen  is  in  a  form  only 
slightly  decomposed  by  steaming.  It  is 
quite  possible  to  procure  and  mix  the 
uncomposted  ingredients,  steam  them, 
and  use  the  soil  for  planting,  all  in  the 
same  day.  This  fact,  plus  the  uniform 
results,  makes  possible  for  the  first  time 
truly  scheduled  production  and  mechani- 
zation. 

In  England,  the  pasteurized  J.  I.  com- 
posts have  been  placed  on  the  market 
by  commercial  suppliers  in  quantities 
varying  from  a  bag  upward.  This  may  be 
the  ultimate  development  with  the  U.  C- 
type  mixes,  with  the  grower  no  more 
"involved  in  the  soil  business"  than  he 
is  presently  in  the  seed  business.  Expe- 
rience in  California  has  shown,  however, 
that  there  are  several  problems  yet  to  be 
solved  before  this  goal  can  be  success- 
fully achieved  here. 

The  Einheitserde 

Still  a  different  solution  of  the  prob- 
lem of  soil  treatment  toxicity  has  been 
developed  in  Germany.  The  Einheitserde 
(Standardized  Soil)  developed  by  Dr.  A. 
Fruhstorfer  of  Hamburg  and  introduced 
in  1948,  is  now  marketed  by  several 
companies  in  that  country.  The  mix  is 


half  peat  and  half  well-aggregated  sub- 
soil clay,  to  which  are  added  ammonium 
sulfate,  superphosphate,  and  potassium 
sulfate.  Half  as  much  fertilizer  is  added 
to  seed  soil  (P-Erde)  as  to  potting  soil 
(T-Erde).  Lime  may  be  added  to  main- 
tain pH  5  to  6.  These  materials  are  mixed 
together  and  used  without  treatment,  be- 
cause the  peat  and  the  subsoil  are  largely 
free  of  weeds,  organisms,  and  decom- 
posable organic  matter.  This  single  mix 
is  used  for  a  wide  range  of  plants. 

This  seems  to  be  a  drastic  method  of 
avoiding  the  treatment-toxicity  problem, 
and  still  involves  the  disadvantages  of 
clay  soils  described  in  this  section,  and 
of  ammonium  accumulation  (Sec.  7). 
The  assumption  is  unwarranted  that  peat 
is  free  of  organisms  capable  of  causing 
disease.  Recent  German  studies  have 
shown  that  the  mix  may  be  infested  with 
pathogens  during  storage  (Danhardt  and 
Ramsch,  1955)  and  handling.  Neither  is 
the  organism  carryover  on  containers 
prevented.  Granting  that  this  method  is 
better  than  using  ordinary  untreated 
composts,  it  still  leaves  much  to  be  de- 
sired. 

In  England,  a  patent  application  for  a 
compost  similar  to  the  Einheitserde  has 
recently  been  published  (Allerton  and 
Ray,  1954).  The  mix  consists  of  1  to  2 
parts  by  volume  of  sphagnum  peat  moss, 
1  to  2  parts  of  fine  vermiculite,  1  to  2 
parts  of  heavy  clay,  plus  fertilizers.  An 
example  of  the  fertilizers  is  to  use  8 
ounces  per  bushel  of  a  mixture  of  2  parts 
by  weight  of  magnesium  sulfate,  2  parts 
potassium  nitrate,  3  parts  ammonium 
sulfate,  and  4  parts  single  superphos- 
phate. This  compost  apparently  is  not 
yet  being  sold. 


SOIL  TOXICITY  IN  RELATION  TO  TREATMENTS 


A  drawback  sometimes  encountered 
from  chemical  or  steam  treatment  of  soil 
il  the  resulting  injury  to  plants  grown  in 
it.  The  type  and  severity  of  the  injury 


varies  with  the  soil,  the  treatment,  the 
plant,  the  time  and  handling  of  the  soil 
after  treatment,  the  environmental  con- 
ditions, and  perhaps  other  factors.  Symp- 


toms  may  be  stunting  of  the  plant,  dis- 
coloration, necrosis,  and  abscission  of 
leaves,  death  of  the  plant,  or  reduced 
seed  germination.  The  toxic  effect  may 
be  temporary  or  last  several  months, 
particularly  if  the  soil  is  kept  sterile  or 
dry.  When  a  proper  soil  mixture  is  used 
this  toxicity  does  not  appear,  and  there 
is  no  reason  to  have  this  problem. 

Substances  Involved 

The  many  investigations  that  have 
been  concerned  with  the  nature  of  the 
toxin  warrant  the  conclusion  that  several 
injurious  agents  may  be  involved.  Briefly 
these  are  as  follows. 

Accumulation  of  ammonium 

Bacteria  that  decompose  ammonium 
are  non-spore-forming  and  more  sensi- 
tive to  heat  and  many  chemicals  than  the 
ammonifying  organisms  which  convert 
organic  nitrogen  to  ammonium.  Treat- 
ments may  thus  cause  the  accumulation 
of  ammonium,  since  there  is  a  delay  in 
conversion  to  nitrate.  As  discussed  in 
Section  7,  accumulation  of  ammonium 
may  reach  toxic  levels  in  2  weeks  and 
last  for  6  to  8  weeks  or  more. 

If  leaching  of  the  soil  reduces  toxicity, 
ammonium  may  not  be  the  factor  in- 
volved because:  if  leaching  is  successful 
soon  after  steaming,  the  ammonium 
usually  has  not  yet  reached  toxic  levels; 
if  leaching  reduces  toxicity  in  soil 
treated  some  weeks  before,  the  am- 
monium may  largely  remain  in  the  soil 
instead  of  being  removed.  Ammonium 
may,  however,  bs  involved  if  prompt 
planting  after  treatment  prevents  injury, 
since  the  transplants  might  be  carrying 
nitrifying  bacteria  that  would  convert 
the  ammonium,  and  the  plants  might 
keep  the  ammonium  at  a  low  level  by 
absorbing  it  as  formed,  both  of  which 
would  prevent  accumulation. 

Water-soluble  organic  matter 

Organic  matter  is  rendered  water- 
soluble    (broken  down)    in  varying  de- 


grees by  heat  and  chemicals,  but  the 
nature  of  the  process  or  the  products  are 
little  understood.  These  materials  are 
removed  by  leaching  immediately  after 
steaming  and  may  be  involved  where 
benefit  is  derived  from  such  treatment. 
Our  experience  has  confirmed  reports 
that  soil  mixtures  high  in  readily  decom- 
posable organic  matter  (manure,  leaf 
mold,  compost,  some  black  peats)  gen- 
erally give  greatest  toxicity  from  steam- 
ing. Highly  organic  soils  also  give 
greatest  residual  toxicity  to  bromine- 
sensitive  plants  (for  example,  carna- 
tions) from  methyl  bromide  fumigation. 

Available  manganese 

Soils,  particularly  of  the  acid  lateritic 
type,  may  release  toxic  amounts  of  man- 
ganese when  steamed.  In  Hawaii  this 
causes  severe  injury  to  some  crops  (let- 
tuce, cowpeas)  in  steamed  soils.  Leach- 
ing of  treated  soils  removes  this  in- 
jurious factor. 

Increase  of  total  soluble  salts 

Some  soil  ingredients  may  release 
enough  adsorbed  salts  when  steamed  to 
produce  plant  injury.  Thus,  our  tests  in 
1944  showed  an  increase  in  conductance 
(EC  x  105  at  25°  C  on  a  2:1  extract) 
from  117  and  151  to  228  and  213,  re- 
spectively, in  two  series  using  a  highly 
organic  mix  plus  cow  manure,  after  45 
minutes'  steaming  at  212°  F.  Leaching 
removes  these  toxic  salts  and  is  com- 
monly practiced  for  this  purpose. 

Other  agents 

Other  agents  that  have  been  reported 
as  resulting  from  soil  steaming  are:  in- 
crease in  water-soluble  salts  of  calcium, 
copper,  magnesium,  potassium,  zinc, 
phosphorus,  and  aluminum;  altered  pH; 
decreased  iron  and  nitrate;  osmotic  con- 
centration of  soil  solution:  modified  ab- 
sorptive capacity  of  soil  for  water,  gases, 
and  salts. 


[95] 


Avoiding   Post-Treatment 
Toxicity 

Several  methods  of  dealing  with  this 
toxicity  problem  have  been  discovered 
and  utilized  under  commercial  condi- 
tions. 

Choice  of  soil  mixture 

The  best  method  is  to  avoid  the  trouble 
by  using  a  soil  mixture  which  does  not 
form  toxins  after  steaming  or  chemical 
treatment  (sees.  5  and  7).  All  the  types 
of  toxicities  resulting  from  heating  soil, 
with  the  possible  exception  of  ammonium 
accumulation,  are  eliminated  by  using  a 
U.  C.-type  soil  mix.  The  ammonium 
problem  may  easily  be  kept  under  con- 
trol by  methods  explained  in  sections  5 
and  7.  The  J.  I.  composts  and  the  Einheit- 
serde  mentioned  earlier  evade  treatment 
toxicity  at  the  expense  of  increased  risk 
from  disease  loss;  the  U.  C.  system  both 
eliminates  the  problem  and  enhances 
protection  from  disease.  Since  there  are 
also  other  advantages  in  a  soil  mix  of 
the  U.  C.  type,  there  is  little  valid  reason 
for  using  a  soil  that  requires  corrective 
measures  for  post-treatment  toxicity.  In- 
terest in  this  toxicity  problem  is  now 
largely  academic,  since  the  choice  of 
proper  ingredients  makes  it  of  no  prac- 
tical concern. 

Leaching  of  the  soil 

Leaching  is  the  present  most  common 
method  of  reducing  soil  toxicity,  and  is 


often  quite  effective.  However,  it  puddles 
the  soil,  creates  a  flood  in  the  green- 
house, removes  soil  nutrients  and  makes 
fertilization  necessary,  increases  the  re- 
contamination  hazard,  delays  planting 
operations,  and  increases  cost  of  labor, 
water,  and  fertilizers.  It  is  an  expensive 
and  messy  solution  of  an  unnecessary 
problem.  It  is  not  very  effective  against 
accumulated  ammonium.  Gypsum  (cal- 
cium sulfate)  may  be  added  to  facilitate 
the  leaching  of  ammonium  from  the  soil 
(Sec.  7). 

Aging  of  the  soil 

Steamed  soil  is  sometimes  left  for 
several  weeks  after  treatment  to  reduce 
toxicity,  presumably  through  reestab- 
lishing a  biological  balance  in  the  soil. 
In  some  cases  it  is  steamed  in  the  fall, 
left  all  winter,  and  used  in  the  spring. 
Among  the  obvious  disadvantages  of  this 
method,  however  effective,  are  the  ex- 
treme recontamination  hazard,  the  delay 
in  operations,  cost  of  storing  idle  soil, 
and  the  expense  of  additional  handling. 

Planting  immediately 
after  steaming 

This  recent  method  may  have  merit 
when  ammonium  accumulation  is  the 
toxic  factor  involved.  There  are,  how- 
ever, some  plants  that  are  sensitive  to 
ammonium  (Sec.  7),  and  these  should 
not  be  used  in  this  way.  See  also  "Ac- 
cumulation of  ammonium,"  above. 


CRITERIA  FOR  PHYSICAL  INGREDIENTS  OF  SOIL  MIXES 


Because  of  the  foregoing  facts,  it  is 
necessary  to  select  ingredients  which  will 
perform  the  required  functions  and 
satisfy  certain  other  practical  and  eco- 
nomic requirements  of  the  growing  op- 
eration. Characteristics  of  each  potential 
component  of  the  soil  mix  are  discussed 
below  and  summarized  in  figure  65.  The 
first   four  characteristics  concern   factors 


which  cannot  be  improved  by  mixing 
with  one  or  more  other  ingredients.  If  a 
component  fails  to  satisfy  any  one  of 
these  conditions  it  is  a  hazard  to  the  use- 
fulness of  the  mix.  The  last  nine  char- 
acteristics are,  however,  subject  to  altera- 
tion or  improvement  by  proper  mixing  of 
ingredients.  Mixtures  of  fine  sand  and 
peat  moss  approach  the  ideal. 


[  %  i 


Characteristics 

E 
o 
o 

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</5 

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c 
0 

l/l 

d) 

c 

LL. 

X 

'e 

0 

2 

Readily  available 
in  uniform  grade? 

!K2g 

1 

Chemically 

uniform? 

Stable  to  steam 
and  fumigation? 

9 

9 

• 

9 

• 

9 

9 

9 

Easily  made  into 
uniform  mix? 

■ 

j 

Good  aeration 
assured? 

? 

■ 

9 

9 

9 

9 

Resistant  to  loss  of 
nutrients  by  leaching? 

9 

Fertility 
low? 

9 

9 

9 

• 

1 

Relatively 
inexpensive? 

Moisture  retention 
reasonably  good? 

9 

Light  in  weight? 
(Low  Density) 

9 

Shrinkage  in  storage 
negligible? 

9 

1 

1 

■ 

9 

Gypsum  or  lime 
required? 

? 

9 

• 

9 

• 

9 

' 

Micronutrients 
adequate? 

•* 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

yes 

unknown  or  unpredictable 
intermediate 


~o  a 


a.  -c  _q  -Q 


I 


4)    O  . — . 

-5  'x  o 

t  -a  -£ 
<u  -a  a 
a.  D  _c 
S  fc-f 

a 


4) 


I 


Fig.  65.  Criteria  for  selecting  the  physical  components  of  the  U.  C.-type  soil  mixes.  Fine  sand 
is  the  best  of  the  inorganic  ingredients.  Peat  moss,  sawdust,  ground  bark,  rice  hulls,  and  shav- 
ings are  best  among  the  organic  constituents. 


Is  it  readily  available 
in  a  uniform  grade? 

There  are  few  inorganic  planting  ma- 
terials which  are  readily  available  in  a 
uniform  grade.  Clays,  silts,  and  loams 
are  apt  to  be  quite  variable  in  particle 
size,  degree  of  aggregation,  and  mineral 
content.  A  unique,  reasonably  uniform 
source  material,  however,  is  naturally 
available  in  the  fine  sand  which  has  been 
deposited  in  certain  areas  by  wind, 
water,  or  both.  Many  of  the  rolling 
coastal  and  interior  California  hills  con- 
tain almost  pure  deposits  of  fine  sand. 
They  are  also  frequently  found  as  sub- 
soils, drift  sands,  desert  sands,  and  lake 
sands.    In   quarry   operations   supplying 


coarse  sand  and  gravel  to  the  construc- 
tion trades,  man  has  often  artificially 
separated  the  other  components  of  a  soil 
from  the  fine  sand  and  left  relatively 
pure  deposits  of  it.  The  separation  is  gen- 
erally made  by  screening  and  water 
washing,  which  remove  the  silt,  clay,  and 
fine  sand  from  the  coarser  materials.  The 
silt  and  clay  float  off,  leaving  deposits  of 
pure  fine  sand.  These  deposits  are  fre- 
quently called  "silt,"  a  misnomer.  Since 
considerable  variation  may  occur  from 
area  to  area,  the  materials  should  be 
carefully  inspected  before  making  a  final 
selection. 

Among  the   organic  physical  compo- 
nents are  various  types  of  peat  moss,  ma- 


[97] 


nure.  leaf  mold,  composts,  and  others 
such  as  the  plant  by-products  (rice  hulls, 
sawdust,  shavings,  ground  bark).  Sphag- 
num peat  mosses  of  the  European  or  Ca- 
nadian type  are  commonly  available  in 
adequate  quantities,  and  in  uniform  and 
reliable  quality.  The  same  cannot  be  said 
of  leaf  mold,  manures,  composts,  and 
some  other  types  of  peat.  Leaf  mold  was 
largely  used  in  the  past,  particularly  by 
growers  in  Europe  and  eastern  United 
States,  because  they  had  access  to  con- 
stant and  readily  available  supplies,  and 
the  material  was  generally  well  decom- 
posed and  leached.  In  many  areas  of 
California  it  is  illegal  to  remove  leaf 
mold  from  its  natural  source.  The  plant 
by-products  mentioned  above  may  not  be 
so  generally  available  but  may  be  fairly 
uniform  in  grade  where  they  are  ob- 
tainable. 

Is  it  chemically  uniform? 

To  be  reliable  as  a  basic  component 
of  the  soil  mix,  each  ingredient  must  be 
reasonably  uniform  in  soluble  and  po- 
tentially soluble  constituents.  Each  in- 
gredient of  a  soil  mix  should  be  non- 
toxic to  plants  if  used  alone.  Again  fine 
sand  and  peat  moss  offer  a  ready  answer, 
for  they  are  both  almost  certain  to  be  low 
in  soluble  constituents  and  relatively  in- 
capable of  release  of  such  materials. 
Other  inorganics  may  or  may  not  have 
these  properties.  Organics  of  the  leaf 
mold,  manure,  and  compost  type  are 
completely  unreliable  in  this  respect. 
These  and  many  of  the  sedge  peats,  such 
as  the  black  peats  from  semiarid  regions, 
have  frequently  been  found  to  be  saline. 
Many  of  the  plant  by-products  are  uni- 
formly low  in  soluble  and  potentially 
soluble  minerals.  On  the  other  hand,  they 
may  actually  tie  up  such  elements  as 
nitrogen  during  their  decomposition  in 
soil. 

Is  it  stable  to  steam  or 
fumigation  treatment? 

Stability  to  treatment  is  one  of  the 
most  important  considerations,  and  was 


one  of  the  primary  reasons  for  the  de- 
velopment of  a  reliable  soil-mix  system. 
Unless  the  mix  is  stable  to  soil  treatment, 
all  the  benefits  obtained  by  destruction 
of  pathogens  may  be  lost  by  plant  dam- 
age from  toxic  materials  produced  in  the 
soil.  The  most  common  toxic  materials 
released  by  such  treatment  are  excessive 
ammonium,  water-soluble  organic  mat- 
ter, manganese,  or  soluble  salts,  as  point- 
ed out  earlier  in  this  section.  This  type  of 
problem  does  not  occur  where  the  soil 
mix  contains  fine  sand,  peat  moss,  saw- 
dust, shavings,  bark,  or  rice  hulls,  but 
may  occur  in  soil  mixes  containing 
loams,  leaf  mold,  manure,  and  composts. 

Is  it  easily  made  into 
a  uniform  mix? 

Ease  of  mechanical  mixing  and  uni- 
formity of  plant  growth  are  important 
considerations  in  the  selection  of  soil  in- 
gredients. Many  growers  in  central  Cali- 
fornia have  purchased  expensive  grind- 
ing equipment  to  break  up  the  clods  in 
clay  soils,  an  operation  eliminated  by 
using  fine  sand.  In  fact,  the  presence  of 
hard  lumps  in  the  soil  is  evidence  that  it 
is  unsuitable  for  use  in  the  mix. 

Organic  materials  such  as  peat  moss 
may  sometimes  be  passed  through  a 
shredder  to  break  up  the  compressed 
bales,  but  this  is  not  an  expensive  or  even 
a  necessary  operation,  for  it  can  be 
broken  up  fairly  readily  by  hand.  Some 
brands  of  Canadian  peat  moss  are  now 
packed  with  a  thin  sheet  of  polyethylene 
as  an  inner  lining  to  the  package.  This 
reduces  water  loss  and  keeps  the  peat  in 
a  slightly  moist  condition,  making  it 
easier  to  break  up  and  mix.  Some  grow- 
ers have  been  using  redwood  fiber,  a 
bark  product,  which  is  difficult  to  make 
into  a  uniform  mix  regardless  of  how  it 
is  handled.  Most  of  the  other  ingredients 
of  a  U.  C.-type  mix  are  easily  blended 
with  one  another. 

Is  good  aeration  assured? 

Because  of  small  pore  size,  clays  and 
sills   lend   to   have  poor  aeration   under 


[98  1 


some  conditions.  The  use  of  organic  mat- 
ter, particularly  peat  moss,  can  greatly 
improve  this  shortcoming.  Fine  sand, 
and  the  coarser  grades  of  sand  and 
gravel  are  individually  well  aerated.  It 
should  be  pointed  out,  however,  that  a 
mixture  of  fine  and  coarse  sands  with 
relatively  small  amounts  of  silt  and  clay 
can  become  very  compact,  and  thus 
poorly  aerated.  This  is  best  illustrated  by 
the  analogy  of  making  concrete:  par- 
ticles of  various  sizes,  ranging  from  sand 
to  gravel  and  rock,  are  mixed  with  water 
and  a  little  cement  (powdery,  claylike) 
to  form  an  impervious  product  upon  set- 
ting. A  product  of  uniform  particle  size 
offers  little  opportunity  for  cementing  ac- 
tion; for  illustration,  one  might  consider 
the  difficulty  of  trying  to  make  concrete 
of  buckshot. 

Most  organic  materials  are  favorable 
to  good  aeration.  If,  however,  these  are 
sufficiently  decomposed,  a  "mucky"  con- 
dition may  develop  which  is  similar  to 
clay  in  its  ability  to  obstruct  air  move- 
ment. Leaf  molds  and  manures  reach  this 
stage  quite  rapidly,  as  compared  with 
peat  moss  or  shavings.  Many  azalea 
growers  are  finding  it  advantageous  to 
include  shavings  in  their  beds  of  peat 
moss  in  order  to  insure  good  aeration  as 
the  peat  decomposes,  and  as  weather  be- 
comes cold  and  wet. 

The  tiny  pores  in  a  clay  or  silt  soil  can 
become  completely  filled  with  water, 
leaving  no  space  for  air.  On  the  other 
hand,  the  considerably  larger  pores  in  a 
sand  or  a  sand  and  peat  moss  mixture 
cannot  be  fully  occupied  by  water  unless 
drainage  is  prevented.  Both  water  and 
air  can  occupy  the  pore  spaces  of  a 
medium  consisting  of  a  uniform  grade  of 
sand.  A  well-aerated  soil  is  a  well-drained 
soil. 

Is  it  resistant  to 
leaching  of  nutrients? 

Loams,  clays,  humus,  and  most  unde- 
composed  organic  materials  are  retentive 
of    certain    elements    of    importance    in 


mineral  nutrition.  This  property  is  one 
of  base  exchange  and  is  peculiar  to  the 
above-mentioned  components.  Ions  such 
as  ammonium,  potassium,  calcium,  and 
magnesium  may  be  held  by  the  particles 
in  available  but  nonleachable  status. 

Another  factor  to  consider  in  leach- 
ability  is  the  amount  of  water  it  takes 
to  actually  displace  the  soil  solution.  In 
comparing  a  silt  and  a  gravel,  it  is  ob- 
viously easier  to  flush  all  soluble  ma- 
terials out  of  the  gravel  than  out  of  the 
silt,  though  neither  has  an  appreciable 
base-exchange  capacity.  Thus,  a  fine 
sand  is  slightly  retentive  of  nutrients  and 
the  coarser  materials  much  less  reten- 
tive. Organic  matter  is  all  more  or  less 
retentive.  A  mix  of  fine  sand  and  peat 
moss  can  be  prepared  to  satisfy  this  re- 
quirement quite  efficiently.  Such  a  mix 
would  have  a  base-exchange  ability  and 
moisture  capacity  comparable  to  a  sandy 
loam  without  the  hazards  introduced  by 
the  clay  content. 

Is  the  fertility  low? 

It  may  seem  unusual  that  there  should 
be  an  advantage  in  using  materials  of 
known  low  fertility.  The  explanation  is 
that  if  fertility  is  low,  it  is  simple  and  in- 
expensive to  add  enough  of  the  correct 
fertilizers  to  bring  it  up  to  the  desired 
level;  whereas  if  fertility  is  high  or  un- 
known, obviously  the  problem  of  what 
and  how  much  to  add  can  be  resolved 
only  by  a  complete  soil  analysis  or  by 
growing  trials.  In  the  case  of  loam, 
clay,  silt,  leaf  mold,  manure,  or  com- 
posts there  is  almost  certain  to  be  an  un- 
known level  of  fertility,  or  even  possible 
salinity.  The  sands  and  peat  mosses  are 
relatively  low  in  fertility.  Wood  shavings 
and  sawdust  are  very  low  in  fertility, 
and  organisms  which  cause  their  decom- 
position utilize  some  or  all  of  that  nor- 
mally supplied.  Ground  bark  has  fre- 
quently been  found  to  be  well  supplied 
with  phosphorus  and  potassium  in  an 
available  form.  Rice  hulls  have  been 
found  to  contain  considerable  potassium 


[99] 


in  soluble  form.  Since  a  mixture  of  fine 
sand  and  peat  moss  provides  a  known 
low  starting  point,  reliability  of  the  final 
mix  is  assured. 

Is  it  relatively  inexpensive? 

Depending  upon  the  value  of  the  crop 
to  be  grown,  the  answer  here  may  be 
quite  variable.  In  general,  the  inorganic 
products  such  as  clay,  loam,  fine  sand,  or 
gravel  are  inexpensive,  hauling  often 
being  the  main  cost.  Peat  moss,  manure, 
leaf  mold,  and  composts  may  be  con- 
siderably more  expensive,  but  far 
from  prohibitive.  Composting  operations 
greatly  increase  the  cost  of  handling. 
Composting  area  further  increases  ex- 
pense by  enlarging  the  space  required  for 
the  growing  operation.  As  mentioned 
earlier,  when  fine  sand  and  peat  moss  are 
used  it  is  possible  to  make  up  the  mix, 
treat,  and  plant  on  the  day  of  delivery. 
The  plant  by-products  such  as  sawdust 
and  shavings  are  fairly  inexpensive, 
sometimes  even  free.  Mixtures  of  fine 
sand  and  peat  moss  are  generally  less  ex- 
pensive than  other  soil  preparations  in 
cost  of  labor. 

Is  moisture  retention 
reasonably  good? 

As  discussed  earlier  in  this  section, 
moisture  supply  is  of  major  importance, 
and  the  ability  of  a  material  to  retain 
moisture  for  a  reasonable  length  of  time 
becomes  important.  Otherwise  too  much 
labor  is  required  for  maintaining  an  ade- 
quate supply  for  plant  growth.  Clays, 
silts,  peats,  and  other  organic  materials 
are  generally  high  in  their  capacity  to 
retain  moisture,  peat  being  highest  of 
all  in  this  property.  Peat  moss  is  a  logical 
material  to  include  in  a  sandy  type  mix 
because  it  increases  water  and  mineral- 
nutrient  retention,  and  further  improves 
aeration. 

Is  it  light  in  weight? 

Current   practices   of   marketing    and 

shipping    plants    in    containers    make    it 


quite  important  that  the  weight  be  kept 
as  low  as  possible.  Light-weight  growing 
media  are  confined  largely  to  the  organic 
materials,  with  peat  moss,  shavings,  and 
sawdust  among  the  lightest.  Sands  and 
gravels  constitute  the  heaviest  materials. 
Perlite  and  vermiculite  are  light-weight 
inorganics,  but  are  rather  expensive.  A 
low-density  growing  medium  is  of  con- 
siderable importance  in  any  operation  in 
which  containers  are  lifted  or  moved 
during  the  growing  period.  A  U.  C.-type 
soil  mix  will  have  about  the  same  density 
as  the  usual  compost  mix,  and  may,  if 
large  proportions  of  peat  are  used,  be 
considerably  lighter. 

Is  shrinkage   in 
storage  negligible? 

Many  materials  have  to  be  held  in 
storage  until  the  grower  is  ready  to  use 
them. 

It  has  been  reported  that  25  to  66  per 
cent  of  the  organic  matter  (and  total 
volume),  50  to  70  per  cent  of  the  nitro- 
gen and  potassium,  and  33  to  50  per  cent 
of  the  phosphorus  are  lost  in  animal  ma- 
nures during  outdoor  storage.  A  ton  of 
fresh  manure  containing  500  pounds  of 
organic  matter,  25  pounds  of  nitrogen, 
25  pounds  of  potash,  and  8  pounds  of 
phosphate  may  produce  1,100  pounds  or 
less  of  material  suitable  for  nursery  use, 
containing  only  250  pounds  of  organic 
matter,  10  pounds  of  nitrogen,  10 
pounds  of  potash,  and  5  pounds  of  phos- 
phate. 

Leaf  mold  in  California  usually  means 
partly  decayed  leaves,  subject  to  a  con- 
siderable but  undetermined  loss  in  com- 
posting. In  New  York  it  was  found  that 
oak  leaves  shrank  33  per  cent,  maple 
leaves  35  to  40  per  cent,  and  wheat  straw 
25  per  cent  during  composting.  Because 
peat  is  largely  decomposed  before  it  is 
dug,  composting  is  unnecessary  and 
shrinkage  during  use  or  storage  is  com- 
paratively minor. 

If  nothing  else,  shrinkage  is  an  eco- 
nomic consideration,  for  it  increases  the 


[100] 


cost  of  the  component  which  is  so  re- 
duced. If  decomposition  is  not  completed 
before  using  the  material,  the  process 
will  continue  in  the  container,  causing 
an  erratic  nutrient  supply  for  the  crop. 
For  this  reason  organic  materials  are 
generally  decomposed  before  use. 

Is  gypsum  or  lime  required? 

Although  gypsum  and  lime  contain 
certain  plant  nutrients,  they  are  usually 
used  as  soil  amendments.  Highly  acid 
materials  such  as  peat  moss  and  wood 
by-products  can  be  neutralized  by  the 
addition  of  the  proper  amount  of  lime.  If 
dolomite  lime  is  used,  both  calcium  and 
magnesium  are  supplied  in  addition  to 
the  neutralizing  action  obtained.  If  the 
component  is  not  too  acid,  it  may  be  de- 
sirable to  use  gypsum  to  supply  calcium 
and  perhaps  reduce  the  potential  danger 
from  alkaline  (high-sodium  or  artificially 


softened)  water.  Mixtures  of  fine  sand 
and  peat  moss  usually  require  the  addi- 
tion of  lime. 

Are  micronutrients  adequate? 

This  is  one  of  the  most  difficult  ques- 
tions to  answer.  These  elements  are  re- 
quired in  such  minute  amounts  that  they 
may  normally  be  supplied  in  adequate 
quantity  by  the  basic  physical  compo- 
nents. Organic  ingredients  had  to  be 
supplied  with  the  elements  in  order  to 
have  developed  in  the  first  place.  In  ad- 
dition, water  used  for  irrigation  may 
supply  some  elements,  and  fertilizers  also 
frequently  carry  micronutrients  as  im- 
purities. Except  for  long-term  growing, 
micronutrients  will  not  usually  constitute 
a  problem.  If  ever  found  to  be  important, 
they  can  be  added  (see  "Micronu- 
trients," below). 


SELECTING  INGREDIENTS  FOR  U.  C.  MIXES 


The  final  mix  may  consist  of  any  one 
or  combination  of  suitable  ingredients. 
//  the  mix  is  a  good  one,  nearly  all  plants 
may  be  grown  in  it.  If  it  is  a  poor  mix, 
only  those  which  are  tolerant  of  the  poor 
features  may  be  grown  in  it. 

The  foregoing  considerations  have 
narrowed  the  choice  of  the  physical  com- 
ponents down  to  a  very  few  which  will 
satisfy  the  requirements.  The  selection 
of  fine  sand  as  an  inorganic  fraction 
seems  obvious.  The  selection  of  the  or- 
ganic fraction  is  confined  to  sphagnum 
peat  moss  or  possibly  some  of  the  other 
materials  mentioned.  It  should  be 
pointed  out  that  some  of  the  artificially 
produced  inorganic  materials  such  as 
perlite  and  vermiculite  also  offer  the  de- 
sirable properties  of  fine  sand.  Their 
cost  is,  of  course,  much  higher,  but  they 
may  have  particular  uses  under  certain 
circumstances.  For  practical  purposes, 
therefore,  the  standard  system  is  based 


on  the  use  of  fine  sand  or  peat  moss  of 
the  sphagnum  type,  or  both. 

Fine  Sand — Specifications 

"Fine  sand"  applies  to  soil  particles  of 
a  certain  size.  As  used  here,  the  maxi- 
mum and  minimum  dimensions  are  0.5 
mm  (approximately  1/50  in.)  and  0.05 
mm  (approximately  1/500  in.),  respec- 
tively. According  to  U.  S.  Department  of 
Agriculture  standards,  this  range  of  par- 
ticle sizes  includes  the  classifications: 
medium  sand,  fine  sand,  and  very  fine 
sand.  It  has  not  seemed  essential  from  a 
practical  standpoint  to  thus  restrict  the 
specification  of  particle  size  limits.  As 
used  here,  therefore,  the  term  fine  sand 
will  be  understood  to  also  include  the 
classifications,  medium  and  very  fine 
sand.  The  relative  diameters  of  the  par- 
ticle sizes  are  shown  diagrammatically 
in  figure  66. 


[101] 


© 


© 


Clay 

(0  -  .002  mm) 

Silt' 

(.002-    .05  mm) 

Very  fine  sand 

(.05-0.1  mm) 

Fine  sand 

(0.1  -0.25  mm) 


Medium  sand 
(0.25  -  0.5  mm) 


Desirable 
particle 
size  range 
for  soil  mix 


Coarse  sand 
(0.5  -1.0  mm) 


Fig.  66.  A  comparison  of  relative  sizes  of  soil  particles,  according  to  the  United  States  Depart- 
ment of  Agriculture  system.  The  circles  represent  the  maximum  and  minimum  for  each  class. 


I   102] 


If  materials  are  being  obtained  from 
gravel  quarries,  the  common  designation 
for  fine  sand  is  "minus  30  and  plus  270." 
These  figures  refer  to  screen  sizes  and 
mean  that  the  material  passes  a  30-mesh 
screen  and  does  not  pass  a  270-mesh 
screen. 

The  identical  grade  or  grades  of  fine 
sand  will  not  be  available  everywhere, 
since  the  degree  of  purity  (freedom  from 
coarser  and  finer  materials)  is  certain 
to  vary.  It  is,  therefore,  necessary  to  set 
up  specifications  or  permissible  percent- 
ages of  particle  sizes  other  than  those 
included  in  this  fine  sand  classification. 

The  least  critical  factor  from  the  stand- 
point of  soil  structure  is  the  amount  of 
material  present  which  is  coarser  than 
the  fine  sand.  If  silt  and  clay  are  present 
in  minimal  amount,  the  inclusion  of 
coarse  sand  and  fine  gravel  will  serve  to 
make  the  mix  more  porous,  heavier,  and 
lower  in  moisture  retention.  Too  coarse 
a  sand  will  permit  the  soil  to  fall  away 
from  the  roots  during  transplanting.  If 
silt  and  clay  are  present  in  quantity,  the 
presence  of  coarser  materials  will  make 
the  mix  more  susceptible  to  compaction 
and  cementlike  properties,  as  already  ex- 
plained. 

The  selection  of  the  proper  soil  source 
by  the  nurseryman  is  of  the  greatest  prac- 
tical importance.  After  he  becomes  ac- 
customed to  the  appearance  and  feel  of 
the  correct  material,  he  may  find  it  fairly 
easy  to  distinguish  good  from  poor 
sources.  To  familiarize  himself  with  the 
proper  type  of  soil,  the  nurseryman 
should  examine  samples  of  suitable  fine 
sand  either  in  use  by  other  growers  or 
exhibited  by  the  Agricultural  Extension 
Service.  Having  found  a  suitable  source, 
a  grower  should  retain  a  sample  for 
future  comparison. 

The  safest  procedure  is  to  actually 
measure  the  percentages  of  the  important 
particle  sizes.  This  type  of  test  is  made 
by  many  commercial  laboratories,  but 
the  requirements  here  are  not  so  exact- 
ing that  a  grower  cannot  carry  them  out 


if  he  desires.  A  high  degree  of  accuracy 
is  not  required  for  practical  purposes. 
The  soil  to  be  used  in  the  tests  should  be 
oven-  or  sun-dried.  The  equipment  re- 
quired is  listed  as  "Soil  Testing  Equip- 
ment" in  the  Appendix. 

For  purposes  of  the  soil  mix  a  de- 
sirable material  will  have  the  following 
limits: 

Coarse  sand — should  not  exceed  12  to 
15  per  cent,  preferably  less. 

Fine  sand — should  not  be  less  than  70 
per  cent,  preferably  85  per  cent  or 
higher. 

Silt  plus  clay — should  not  exceed  15 
per  cent,  preferably  lower. 

The  results  of  physical  analysis  of  a 
number  of  sandy  soils  which  have  been 
in  use  are  given  in  table  10. 

The  basic  soil  should  be  low  in  organic 
matter,  because  (1)  toxins  are  not  then 
produced  on  steaming,  (2)  it  supplies  a 
known  base  to  start  from  in  adding  peat, 
(3)  the  organic  fraction  might  throw  off 
the  determinations  of  particle  size. 

Determination  of  coarse 
sand  and  gravel 

1.  Weigh  100  grams  of  soil  into  a  quart  jar, 
add  a  pint  of  tap  water,  and  shake  vigor- 
ously to  disperse  the  fine  materials. 

2.  Obtain  the  dry  weight  of  the  sieve  (32 
mesh,  or  0.5  millimeter  diameter  holes) 
and  then  wash  the  dispersed  soil  through 
the  sieve.  Use  a  gentle  flow  of  water  from 
a  tap  and  gently  swirl.  Avoid  flushing  soil 
over  the  edge  of  the  sieve. 

3.  When  no  more  material  will  come  through 
the  sieve,  dry  it  in  an  oven  or  in  the  sun 
and  reweigh  sieve  and  contents. 

4.  The  percentage  of  coarse  sand  and  gravel 
is  equal  to  the  gain  in  weight  in  grams  of 
the  sieve  with  contents. 

Determination  of  silt  and  clay 

1.  Fill  the  hydrometer  jar  about  half  full  of 
distilled  water  and  add  10  milliliters  of 
saturated  sodium  oxalate  solution.  This 
saturated  solution  is  prepared  as  follows: 
To  a  pint  jar  partly  full  of  distilled  water 
add  V-i  pound  of  sodium  oxalate.  Shake 
occasionally  over  a  period  of  several  days 
before  use.  Use  the  clear  liquid  from  the 


[  103  ] 


Table  10.  Physical  Analysis  of  Sandy  Soils  Used  by  Nurserymen 


Per  cent  of  various  particle  sizes 

Desirability 

Soil  sample  No. 

Coarse 
particles 

Fine 
sand 

Silt  plus 
clay 

for 
soil  mix 

1 

2 

3 

4 

4.1 
3.7 
7.3 
4.6 
42.1 
10.0 
3.7 

92.4 
76.1 
82.7 
91.7 
32.9 
71.8 
90.7 

3.5 
20.2 
10.0 

3.7 
25.0 
18.2 

5.6 

Excellent 
Poor 
Good 
Excellent 

5 

6 

Very  poor 
Fair 

7 

Excellent 

No.  1.  Field  soil  with  good  drainage  and  no  compaction.  Coastal  bluff  in  Santa  Barbara  area. 

No.  2.  Field  soil  from  same  area  as  no.  1,  but  with  poor  drainage  and  a  tendency  to  form  compacted  zones 

unless  large  amounts  of  organic  matter  are  used. 
No.  3.  "Silt"  from  a  quarry  operation  in  the  Monrovia  area.  The  washing  process  results  in  the  removal  of 

most  of  the  coarse  materials  for  sale  as  building  sands  and  the  clay  is  washed  away.  The  so-called 

"silt"  or  waste  material  is  actually  fine  sand.  Mixes  made  from  this  product  are  of  good  physical  texture. 
No.  4.  Deposits  of  wind-blown  origin.  Large  hills  in  Palos  Verdes  area.  Also  obtainable  as  waste  "silt"  from 

local  quarry  operations  in  this  area. 
No.  5.  A  soil  from  the  Redondo  area  which,  when  used  in  the  soil  mixes,  gave  unsatisfactory  results  because 

of  a  tendency  to  set  like  concrete.  Note  the  particle  size  distribution. 
No.  6.  A  top  soil  from  the  Torrance  area.  Gave  good  results  with  high  peat-moss  content,  but  not  quite  so  good 

with  mixes  low  in  organic  matter. 
No.  7.  A  wind-blown  sand  from  the  Colma  (San  Francisco)  area.  Excellent  structure  in  the  soil  mixes. 


top  of  the  jar.  Distilled  water  may  be 
added  to  replace  that  used  so  long  as 
crystals  are  still  present  in  the  jar. 

2.  Weigh  another  100  grams  of  the  dry  soil 
and  pour  into  the  jar.  Allow  to  stand  for 
5  or  10  minutes. 

3.  Disperse  the  soil  in  the  water  by  placing 
the  palm  of  the  hand  over  the  mouth  of  the 
jar  and  inverting.  Repeat  this  several  times 
in  a  period  of  20  minutes.  If  necessary,  add 
more  sodium  oxalate  to  keep  the  silt  and 
clay  fraction  dispersed  after  mixing.  When 
properly  dispersed  the  liquid  will  remain 
quite  muddy  after  standing  about  5 
minutes. 

4.  Add  distilled  water  to  the  hydrometer  jar 
until  the  total  volume  of  liquid  is  up  to  the 
1,000-miIliliter  mark. 

5.  Invert  the  jar  several  times  and  quickly 
place  it  upright  on  a  level  bench  and  note 
the  time  on  the  second  hand  of  a  watch. 

6.  Place  the  hydrometer  gently  in  the  column 
and  allow  it  to  come  to  equilibrium.  At 
exactly  40  seconds  after  setting  the  jar 
upright  take  the  reading  of  the  liquid  level 
on  the  hydrometer  stem.  This  reading, 
when  corrected  for  temperature,  is  the  per- 
centage of  silt   phis  clay. 

7.  After  taking  the  hydrometer  reading  and 
removing  the  hydrometer,  measure  the 
temperature  of  the  liquid  in  the  jar.  For 
<  \ery  degree  above  67°    F,  add  0.2  to  the 


reading  obtained  on  the  hydrometer.  For 
every  degree  below  this  temperature,  sub- 
tract 0.2. 

Determination  of  fine  sand 

The  percentage  of  fine  sand  can  be  calculated 
from  the  data  obtained  above.  The  fine-sand 
fraction  is  equal  to  the  sum  of  the  two  deter- 
mined fractions  (coarse  sand  and  gravel,  plus 
silt  and  clay)   subtracted  from  100. 

The  hydrometer  test  outlined  above  is  subject 
to  increasing  error  as  the  silt,  clay,  and  organic 
content  of  the  soil  being  tested  increase. 
Ordinarily,  the  fine  sands  are  low  enough  in  all 
of  these  to  make  such  errors  negligible. 

Peat  Moss 

The  only  other  basic  component  of  the 
standard  mixes  is  peat  moss  of  the  sphag- 
num type  obtained  from  Canada,  Europe, 
and  some  parts  of  northern  United 
States.  California  hypnum  peat  moss  has 
also  proved  to  be  satisfactory.  In  general, 
other  types  of  peat  are  too  uncertain  in 
chemical  and  physical  composition  to  be 
included  in  standard  mixes.  For  instance, 
many  of  the  sedge  peats  of  arid  areas 
(for  example,  the  black  peat  of  coastal 


[104] 


California )  are  saline.  Some  are  infested  gins.  Unless  the  soil  is  put  to  use  within 

with  undesirable  organisms.  a  week  the  nitrogen  released  may  become 

The  peat  moss  should  be  finely  ground  excessive    for    plant    growth.    Materials 

so  that  it  will  readily  mix  with  the  other  which  have  been  used  as  organic  sources 

component,  fine  sand.  The  mixing  opera-  of  nitrogen  in  the  U.  C.  system  of  prepa- 

tion  is  greatly  facilitated  by  moistening  ration  are: 

the  peat  a  day  or  two  beforehand.  Approximate  per  cent 

Material  of  nitrogen 

—■       B       •     ^»i_         •   _i  ■_       ^  j:^_*.  Hoof  and  horn  meal   13 

The  Basic  Chemscal  ingredients  D1      ,         ,  ... 

Blood  meal    lo 

In  order  that  plants  may  grow  prop-  Cottonseed  meal 7 

erly,  mineral  nutrients  must  be  included  Castor  pomace  . . 6 

,  .  ,.  rpii      i       •       l       •  Urea-formaldehyde  resin    So 

in  the  growing  medium.  Ihe  basic  physi- 
cal  ingredients   that   have   been   recom-  Anyone  contemplating  the  use  of  urea 

mended  are  almost  certain  to  be  low  in  or  urea-formaldehyde  fertilizers  should 

the     required     elements,     providing     a  note  the  warning  about  possible  biuret 

known  starting   point.  The  problem   of  injury  on  p.  79. 

providing  a  medium  containing  a  rea-  There   may   be   other   acceptable   ma- 

sonable  balance  of  required  nutrients  is  terials,  but  so  far  they  have  not  been  suf- 

easily  met  by  the  addition  of  a  few  com-  ficiently  tested  to  justify  their  inclusion. 

mon  fertilizers.  The  material  most  used  so  far  has  been 

hoof  and  horn  meal. 
Nitrogen  There  is  little  point  in  adding  the  am- 

The  proper  use  of  nitrogen  is  probably  monium  form  of  nitrogen  to  the  mix.  As 
the  most  important  factor  in  the  nutri-  explained  in  Section  7,  this  form  is  toxic 
tional  success  or  failure  of  this  or  any  to  some  plants  when  present  in  sufficient 
system  of  soil  mixes.  A  more  complete  quantity.  Further,  it  will  be  present  in 
description  of  the  complexities  is  pro-  some  quantity  where  organic  nitrogen  is 
vided  in  Section  7.  Briefly,  nitrogen  can  supplied,  since  it  is  the  first  major  prod- 
be  supplied  in  any  one  of  three  forms,  or  uct  of  organic-nitrogen  conversion, 
their  combinations  —  organic,  ammo-  In  some  cases  it  may  be  desirable  to 
nium,  or  nitrate.  supply  nitrogen  in  the  mix  in  the  nitrate 

Organic  forms  are  desirable  from  the  form.  This  can  be  done  by  using  any  one 

standpoint  of  providing  slowly  available  of  several  materials;  typical  sources  are 

nitrogen  over  an  extended  period  of  time,  calcium  nitrate,   potassium  nitrate,  and 

perhaps  making  it  unnecessary  to  apply  sodium   nitrate.   Each   of  these   sources 

additional  nitrogen  during  the  period  of  must  necessarily  involve  the  addition  of 

release.   The   rate   of   breakdown   is   de-  another  element.  Since  potassium  must 

pendent    upon    many    factors,    such    as  be  added  anyway,  the  obvious  choice  is 

temperature,  type  and  population  of  soil  potassium  nitrate.  Because  the  amount  to 

organisms   (sees.  7  and  14),  nature  of  be  added  is  small  in  relation  to  the  bulk 

material  supplied,  and  type  of  soil  treat-  of  the  mix,   it  is  frequently  good  pro- 

ment.  For  a  full  explanation  of  the  bases  cedure  to  dissolve  the  salt  in  water  and 

for  selection  of  materials  and  methods  sprinkle  this  solution  over  the  soil  before 

of  use  see  Section  7.  Since  the  use  of  too  or  during  mixing. 

much  of  these  materials  may  result  in  Another  procedure  sometimes  used  is 
poor  plant  development,  it  is  desirable  to  omit  all  nitrogen  sources  from  the  mix 
to  make  trial  runs  for  each  major  crop  and  then  start  a  program  of  liquid  or  dry 
before  deciding  the  amount  to  be  in-  surface  feeding  immediately  after  plant- 
eluded.  Once  the  organic  nitrogen  has  ing.  Such  a  program  might  consist  of 
been  added  to  the  soil,  breakdown  be-  liquid  feeding  with  a  solution  made  up 

[105] 


of  some  soluble  nitrogen  source  such  as 
calcium  nitrate,  ammonium  nitrate,  am- 
monium sulfate,  or  urea  (Sec.  5).  Dry 
application  to  the  soil  surface  of  hoof 
and  horn  or  blood  meal  might  be  used 
instead  of  a  liquid  source. 

Phosphorus 

Of  the  several  sources  of  available 
phosphorus,  those  which  best  suit  the 
purpose  here  are  known  as  superphos- 
phates. There  are  two  principal  grades, 
both  made  from  phosphate  rock  and  both 
supplying  readily  available  phosphorus 
with  a  low  salinity  effect.  They  are 
known  as  single  superphosphate  (18  to 
20  per  cent  available  P205)  and  double 
superphosphate,  also  known  as  treble  or 
triple  superphosphate  (40  to  50  per  cent 
available  P205) .  These  are  monocalcium 
phosphates,  the  former  containing  extra 
calcium  in  the  form  of  gypsum. 

Potassium 

As  indicated  above,  this  element  can 
be  supplied  in  the  form  of  potassium 
nitrate  (46  to  48  per  cent  K20).  Other 
sources  are  potassium  sulfate  (50  per 
cent  K20)  and  potassium  chloride  (60 
per  cent  K20) . 

All  sources  of  this  element  currently 
in  common  use  are  completely  water- 
soluble  and  must  be  used  with  care,  as 
they  add  directly  to  the  soluble  salts  of 
the  soil  mix.  The  sulfate  form  is  most 
frequently  used,  the  sulfur  fraction  also 
being  a  plant  nutrient.  A  slowly  available 
form  of  potassium  would  be  an  ad- 
vantage. Glass  frits  containing  12  and  20 
per  cent  potash  prepared  by  the  Ferro 
Corporation,  Cleveland,  Ohio,  were  used 
experimentally  by  the  Department  of 
Plant  Pathology  and  R.  H.  Sciaroni  in 
1951-52.  Manufacturing  difficulties  kept 
this  product  ofT  the  market.  A  35  per  cent 
potassium  frit  (Dura-K)  has  recently  be- 
come available. 

Lime 

Almost  any  proportions  of  the  fine 
sand  and  peat  moss  will  result  in  a 
medium    which    is  acid    in    reaction.  The 


fine  sand  usually  will  have  very  little 
buffering  capacity,  even  if  it  is  alkaline 
in  reaction.  The  highly  acid  peat  moss 
has  a  high  buffering  capacity  and  will 
be  a  predominant  factor  in  the  final  pH 
reaction.  A  desirable  pH  range  for  the 
growth  of  most  green  plants  is  between 
5.5  and  6.5.  The  pH  of  peat  moss  is  nor- 
mally in  the  vicinity  of  4.0.  In  order  to 
partially  neutralize  the  acidity,  it  is  neces- 
sary to  add  some  form  of  lime.  Calcium 
carbonate  or  oystershell  lime  are  typical 
of  acceptable  materials  which  will  supply 
calcium  and  also  neutralize  the  acidity. 
Dolomite  lime  is  a  natural  form  of  cal- 
cium and  magnesium  carbonate,  which 
supplies  two  elements  important  to  plant 
growth,  calcium  and  magnesium,  and 
which  also  provides  the  necessary  neu- 
tralizing effect. 

Some  fine  sand  sources  may  possibly 
contain  lime,  and  in  such  cases  the  lime 
addition  to  the  mix  should  be  adjusted 
for  this.  Otherwise  the  final  reaction  of 
the  mix  might  be  high  enough  to  result 
in  some  micronutrient-deficiency  symp- 
toms in  sensitive  plants.  Because  such 
a  situation  is  rare,  the  recommendations 
are  based  on  the  assumption  that  no  lime 
is  present. 

The  calcium  and  magnesium  addition 
may  be  important  in  cases  where  irriga- 
tion water  is  either  extremely  low  in 
salinity  or  where  it  is  soft  owing  to  a 
high  sodium  percentage.  If  extra  calcium 
is  considered  desirable,  it  may  be  sup- 
plied as  gypsum  without  affecting  soil  re- 
action. Where  waters  are  hard,  but  low 
in  sodium  percentage,  there  is  little 
danger  of  deficiency  of  calcium  and 
magnesium. 

Micronutrients 

Mineral  elements  necessary  to  plant 
growth  and  development  include  those 
previously  mentioned  (nitrogen,  phos- 
phorus, potassium,  calcium,  magnesium, 
sulfur)  plus  certain  others  (zinc,  manga- 
nese, iron,  boron,  molybdenum,  copper) 
which   are  termed  "micronutrients"  be- 


I  L06  | 


cause  their  presence  is  necessary  in  only 
minute  amounts.  In  fact,  they  can  be 
quite  toxic  to  plants  if  present  in  more 
than  a  few  parts  per  million  parts  of  soil. 
The  fact  that  they  are  required  in  such 
minute  amounts  and  are  natural  compo- 
nents of  peat,  soil,  fertilizers,  and  water 
makes  it  improbable  that  a  soil  mix  will 
have  a  deficiency  of  a  minor  element. 

Investigations  to  date  have  shown  no 
benefit  from  micronutrient  additions  to 
the  U.  C.  soil  mixes,  except  in  special 
cases  such  as  the  following.  It  has  become 
standard  practice  in  growing  certain 
crops  to  apply  some  of  the  chelated  iron 


products  during  certain  periods  of  the 
year,  usually  winter.  Azaleas,  gardenias, 
roses,  Liquidambar,  heather,  and  sanse- 
vieria  are  examples  of  crops  commonly 
receiving  this  type  of  treatment  in  Cali- 
fornia nurseries.  Frequently  the  iron- 
chlorosis  pattern  on  the  foliage  is  due  to 
root  disease  (Sec.  3),  excess  fertilizer, 
or  excess  lime,  and  can  therefore  be 
eliminated  by  the  use  of  soil  mixes  and 
handling  procedures  outlined  in  this  pub- 
lication. Until  further  investigations  in- 
dicate some  benefit  from  micronutrient 
additions,  no  general  recommendation 
for  their  use  is  made. 


[107] 


SECTION 


Nitrogen  in 
Nursery  Soils 


O.  A.  Matkin 
Philip  A.  Chandler 

Types  of  nitrogen 

The  ideal  soil  mix 

Tests  with  nitrogen  in  the  U.C.  soil  mixes 

Factors  affecting  nitrogen  release  from  organic  sources 

Surface  application  versus  mixing 

Adjusting  to  specific  situations 


lthough  NO  one  essential  nutrient 
is  more  important  than  any  other  to 
plants,  certain  elements  assume  greater 
importance  because  of  practical  limita- 
tion of  supply.  Nitrogen,  phosphorus, 
and  potassium  receive  the  most  attention 
in    the    average    fertilizer    program.    Of 


these  three,  nitrogen  is  most  complex  in 
form  and  behavior  in  both  soil  and  plant. 
In  the  average  nursery  or  greenhouse  op- 
eration, adequate  control  of  nitrogen 
availability  constitutes  the  major  fertil- 
izer problem. 


TYPES  OF  NITROGEN 


For  purposes  of  this  section,  nitrogen 
may  be  considered  as  existing  in  three 
general  forms. 

Organic  nitrogen 

Organic  nitrogen  is  generally  unavail- 
able for  plant  use.  An  important  excep- 
tion is  urea,  which  is  organic  but  for  all 
practical  purposes  acts  like  an  inorganic 


Urea  and  urea-formaldehyde  fertilizers 
may  contain  biuret,  a  by-product  toxic 
to  many  plants.  Unless  labeled  biuret- 
free,  these  materials  should  be  used  only 
after  thorough  testing  on  each  crop. 


form.  Organic  nitrogen  is  linked  to 
carbon  and  is  not  normally  water- 
soluble.  Certain  organic  forms  such  as 
the  amino  acids  (structural  components 
of  the  proteins)  have  been  shown  to  be 
absorbed  by  plant  roots,  but  their  oc- 
currence and  importance  in  the  nursery 
soil  is  minor. 

Insoluble  organic  nitrogen  might  be 
considered  as  a  storage  form;  available 
nitrogen  will  be  produced  from  it  by 
chemical  or  microorganism  activity. 
Sometimes,  as  in  manures  or  leaf  molds, 
steaming  will  result  in  a  chemical  reac- 
tion which  produces  an  available  form. 
The  more  stable  organics  will  require  ac- 


0 


tion  by  microorganisms  to  produce  this 
available  nitrogen. 

Ammonium  nitrogen 

The  first  breakdown  product  of  or- 
ganic nitrogen  which  is  of  practical  in- 
terest to  the  nurseryman  is  ammonium. 
This  ammonium  nitrogen  may  also  be 
provided  as  a  commercial  fertilizer,  such 
as  ammonium  sulfate  or  dissolved  am- 
monia. Urea  is  quickly  hydrolyzed  in 
the  soil  to  produce  the  ammonium  form. 

Ammonium  nitrogen  is  water-soluble, 
can  be  absorbed  readily  by  the  roots,  and 
can  be  utilized  by  the  plant.  In  certain 
cases  utilization  may  be  so  fast  as  to  dis- 
rupt normal  functioning  of  the  plant.  Al- 
though water-soluble,  the  ammonium 
form  of  nitrogen  is  not  as  readily  leached 
through  the  soil  as  is  nitrate.  Because  of 
the  positive  charge  of  the  ammonium  ion 
it  may  be  removed  from  solution  by  the 
negatively  charged  clay  and  organic  in- 
gredients; this  prevents  its  being  leached 
from  the  soil.  Nitrate,  which  is  negatively 
charged,  may  be  readily  leached.  This 
nonmobile  status  of  ammonium  is  tem- 
porary, owing  to  its  subsequent  conver- 
sion to  nitrate.  Also,  the  presence  of 
other  positively  charged  ions  (for  ex- 
ample, calcium,  magnesium,  sodium,  and 
potassium)  may  cause  the  ammonium  to 
be  "unseated"  and  released  into  the  soil 
solution,  which  makes  it  leachable.  It  is 
evident  from  the  foregoing  that  am- 
monium nitrogen  applied  to  the  soil  sur- 
face may  not  reach  the  root  zone  with 
the  first  irrigation.  For  this  reason  it  is 
sometimes  referred  to  as  being  more 
slowly  available  than  the  nitrate  form, 
whereas  it  is  just  as  available  as  nitrate 
if  it  is  in  the  vicinity  of  the  root. 


Nitrate  nitrogen 

The  conversion  of  ammonium  nitrogen 
to  nitrate  nitrogen  is  also  by  microor- 
ganisms (Sec.  14).  Nitrate  is  completely 
water-soluble  and,  since  it  is  negatively 
charged,  it  does  not  enter  into  any  non- 
mobile  combinations  with  soil  compo- 
nents. In  a  normal  well-aerated  soil, 
nitrate  is,  from  a  practical  viewpoint, 
the  end  product  of  nitrogen  conversions. 
Under  prolonged  conditions  of  compost- 
ing of  organic  products,  most  of  the 
nitrogen  will  be  converted  to  the  nitrate 
form.  In  the  field  little  ammonium  nor- 
mally exists  except  under  very  acid  or 
waterlogged  conditions,  the  organic  ni- 
trogen being  slowly  converted  to  am- 
monium and  then  relatively  rapidly  to 
nitrate.  Under  the  natural  conditions 
where  plants  evolved,  nitrate  was,  there- 
fore, the  principal  form  of  nitrogen 
utilized.  Perhaps  some  plants  have  lost 
their  tolerance  to  ammonium  as  a  result. 

The  cycle  of  nitrogen  within  the  plant 
is  the  opposite  of  that  in  the  soil.  Nitrate 
is  slowly  converted  to  ammonium,  which 
is  rapidly  combined  with  certain  or- 
ganic compounds  to  form  proteins,  en- 
zymes, pigments,  and  many  other  com- 
plex substances.  Under  natural  condi- 
tions, ammonium  nitrogen  and  its  first 
reaction  products  will  not  be  present 
within  the  plant  in  appreciable  quan- 
tities. Under  artificial  conditions,  if  am- 
monium nitrogen  is  supplied  directly,  an 
abnormal  accumulation  of  ammonium 
and  initial  reaction  products  may  occur 
within  the  plant,  resulting  in  possible 
"self-poisoning."  An  important  concern 
of  this  section  is  this  ammonium  toxicity 
and  its  prevention. 


THE  IDEAL  SOIL  MIX 


The  perfect  soil  mix  would  not  only  conditions  during  plant  growth.  This 
have  the  desired  physical  properties  but  state  would  presumably  be  provided  by 
would  also  maintain  optimum  chemical      incorporating  in  the  mix,  sources  of  ni- 

[109] 


trogen,  phosphorus,  potassium,  and  the 
other  elements,  so  that  an  optimum 
amount  would  be  available  for  initial 
plant  growth.  As  plant  growth  and  leach- 
ing reduced  the  available  nutrients  they 
would  be  supplied  by  breakdown  of  some 
insoluble  storage  form.  The  U.  C.-type 
soil  mixes  have  been  an  effort  to  ap- 
proach this  ideal  as  closely  as  is  prac- 
ticable. To  provide  an  indefinite  period 
of  optimum  fertility  under  a  variety  of 
growing  conditions  is  impossible  with 
present  knowledge  and  materials.  The 
first  compromise  must,  therefore,  be  to 
provide  a  limited  period  of  optimum  fer- 
tility and  rely  on  customary  procedures 
of  fertilization  to  extend  the  useful  fer- 


tility of  the  mix.  In  the  case  of  most  ele- 
ments this  is  readily  accomplished;  phos- 
phorus, potassium,  calcium,  and  magne- 
sium can  be  supplied  (Sec.  5),  and  they 
will  remain  in  adequate  supply  for  a  con- 
siderable period  of  time.  The  micro- 
nutrients  seem  to  be  supplied  by  the  raw 
materials  of  a  U.  C.-type  mix  (Sec.  6). 
The  principal  problem  is  that  of  the 
nitrogen  supply.  The  insoluble  and  initi- 
ally unavailable  organic  form  of  nitro- 
gen offers  an  opportunity  to  provide  the 
desired  reservoir.  The  addition  of  a 
small  amount  of  initially  soluble  nitro- 
gen (for  example,  as  nitrate  from  po- 
tassium nitrate)  would  seem  to  satisfy 
the  requirements. 


TESTS  WITH  NITROGEN  IN  THE  U.  C.  SOIL  MIXES 


Seldom  can  a  procedure  or  theory  be 
developed  for  general  practical  use  by 
research  confined  to  the  laboratory.  The 
U.  C.-type  soil  mixes  were  no  exception. 
Procedures  which  seemed  to  work 
out  very  satisfactorily  in  the  Department 
of  Plant  Pathology  glasshouses  some- 
times failed  in  nurseries.  In  the  process 
of  development,  advantage  was  taken  of 
these  failures  to  determine  the  shortcom- 
ings both  of  the  mixes  and  of  the  aver- 
age nursery.  Some  of  the  problems  en- 
countered were  related  to  the  physical 
aspects  of  the  medium  and,  as  a  result  of 
information  gained  from  their  study, 
rather  definite  specifications  have  been 
outlined  in  Section  6.  Fertility  problems 
seemed  to  be  related  primarily  to  the 
quantity  and  form  of  nitrogen  present. 

Our  first  attempts  to  provide  a  stand- 
ard formula  for  fertilizer  additions  fol- 
lowed the  important  contributions  of  the 
John  Innes  Horticultural  Institution  in 
that  the  only  nitrogen  addition  was  hoof 
and  horn  meal.  This  material  would  pro- 
\  ide  a  continuous  supply  of  nitrogen  that 
mi^'lit  almost  match  the  progressively  in- 
creasing   requirements   of   the   growing 


plant.  Initial  trials  indicated  that  this 
procedure  did  work  quite  well.  The  John 
Innes  rates  of  organic  nitrogen  were  in- 
creased substantially,  which  resulted  in 
improved  plant  growth;  this  was  because 
the  fine  sand  is  much  lower  in  nitrogen 
than  the  composted  turf  loam  used  by 
the  Institution. 

The  resulting  procedures  were  adopted 
by  a  few  growers  who  were  having  dif- 
ficulties with  compost-type  mixes.  Most 
of  these  growers  produced  bedding 
plants,  excellent  crops  for  testing  a  soil- 
mix  procedure  because  the  plants  are  in 
the  most  sensitive  stage  of  development, 
and  results  are  quickly  evident.  There- 
fore, most  of  the  studies  reported  in  this 
section  were  conducted  on  bedding- 
plant  problems. 

Relatively  few  growers  were  treating 
their  soils  for  disease  control  at  this 
time,  and  the  first  results  were  very  en- 
couraging. However,  several  shortcom- 
ings soon  became  apparent.  First,  disease 
still  frequently  wiped  out  substantial 
portions  of  the  crop;  second,  initial 
growth  of  the  transplanted  seedlings  was 
not  always  as  rapid  as  was  observed  in 


[110] 


some  of  the  compost  mixes.  The  first 
difficulty  could  be  prevented  by  treat- 
ment of  soil,  containers,  and  planting 
material,  and  a  sanitation  program  (sees. 
1,  3,  and  8  through  13).  The  standard- 
ized system  of  soil  mixes  was  being  de- 
veloped to  make  such  treatment  simple 
and  certain.  Comparative  soil  tests  sug- 
gested that  the  second  problem  was  re- 
lated to  available  nitrogen  supply,  and 
studies  on  this  were  undertaken. 

Tests  with  Starter  Solutions 

We  used  two  materials,  ammonium 
sulfate  and  calcium  nitrate,  at  several 
strengths  as  liquid  fertilizers.  Seedlings 
were  transplanted  to  flats  containing  soil 
mix  B  with  fertilizer  V  (B)  or  VI  (B) 
(Sec.  5)  and  watered  in  with  these  solu- 
tions. No  subsequent  fertilizer  was  ap- 
plied. 

The  results  of  these  trials  indicated 
that  calcium  nitrate  at  approximately  2 
ounces  per  5  gallons  of  water  would 
greatly  enhance  the  initial  growth  with 
little  danger  of  plant  damage.  Ammo- 
nium sulfate  was  ineffectual  or  retard- 
ing, and  was  not  used  in  further  tests. 

Extensive  experiments  were  conducted 
in  flats  in  the  glasshouse,  using  twenty 
kinds  of  bedding  plants  and  vegetables. 
Seedlings  of  fifteen  of  them  (China  aster, 
tomato,  zinnia,  stock,  Tagetes,  pansy, 
verbena,  phlox,  celery,  lobelia,  pepper, 
snapdragon,  delphinium,  vinca,  and 
cineraria)  had  87  to  100  per  cent  (aver- 
age 97.6  per  cent)  survival  when  sup- 
plied with  ammonium  from  hoof  and 
horn  meal  in  high  or  low  amounts — 
fertilizers  V  (B)  or  VI  (B) — with  or 
without  a  calcium  nitrate  starter  solu- 
tion. Seedlings  of  clarkia,  sweet  alyssum, 
Iceland  poppy,  and  carnation  had  poor 
survival  (0  to  80  per  cent;  average,  50.3 
per  cent)  when  supplied  with  high  or 
low  amounts  of  hoof  and  horn  but  no 
calcium  nitrate,  as  against  47  to  97  per 
cent  (average,  79.3  per  cent)  with  cal- 
cium nitrate.  In  every  instance  but  one, 


growth  (fresh  weight)  was  greater  when 
calcium  nitrate  was  added  to  either  low 
or  high  amounts  of  hoof  and  horn. 
Growth  was  superior  in  half  the  cases 
with  low,  and  half  with  high  amounts 
of  hoof  and  horn,  with  or  without  cal- 
cium nitrate.  It  was  concluded  that  the 
calcium  nitrate  starter  solution  was 
beneficial  to  all  the  plants  tested,  and 
was  extremely  helpful  with  a  few. 
Clarkia,  sweet  alyssum,  and  carnation 
appeared  to  be  sensitive  to  ammonium 
(exhibiting  marked  reduction  in  survival 
and  growth),  and  this  was  partially  pre- 
vented by  the  calcium  nitrate  (fig.  67). 
If  the  plant  immediately  began  rapid 
growth  it  was  able  to  use  ammo- 
nium, but  if  it  was  growing  slowly  am- 
monium seemed  to  depress  growth. 

Soon  after  the  above  tests  were  con- 
ducted, nursery  use  of  the  calcium  nitrate 
solution  was  successfully  undertaken.  It 
was  a  logical  deduction  that  the  nitrate 
might  be  supplied  in  the  mix  by  sub- 
stituting potassium  nitrate  (fertilizers  I, 
II,  III)  for  the  previously  used  potas- 
sium sulfate  (fertilizers  IV,  V,  VI).  This 
procedure  was  adopted  by  some  nur- 
series, and  eliminated  the  starter  solu- 
tion. 

Ammonium  Toxicity 

The  solution  of  one  problem  seemed 
to  lead  to  another.  In  certain  crops  at 
times  during  the  year,  plant  stunting 
and  loss  occurred.  Root  damage  was  ob- 
served which  did  not  involve  a  pathogen. 
In  some  crops  leaf  burning  indicated 
possible  saline  conditions.  Comparative 
soil  tests  indicated  that  in  only  a  few 
cases  was  salinity  a  factor,  but  that  am- 
monium was  always  present  in  substan- 
tial amounts  with  little  or  no  nitrate 
nitrogen.  The  most  severe  damage  oc- 
curred when  ammonium  was  present  in 
sufficient  quantity  to  produce  a  saline 
condition.  In  some  seed  flats,  yellowing 
of  seedlings  occurred  when  ammonium 
was  the  predominant  source  of  nitrogen, 
even  though  present  in  relatively  small 


[in] 


4tto 


Fig.  67.  Beneficial  effect  of  a  calcium  nitrate  starter  solution  on  seedlings  of  stock  (above)  and 
petunia  (middle)  grown  in  steamed  U.  C.  soil  mix  B  (25  per  cent  peat;  IVi  lb.  of  hoof  and 
horn  meal  per  cubic  yard).  Flats  at  left  received  single  applications  of  the  solution,  those  at 
the  right  did  not.  Ammonium  injury,  shown  by  sweet-alyssum  seedlings  (below,  left)  grown  in 
the  same  mix,  was  reduced  by  three  applications  of  calcium  nitrate  solution  (below,  right). 


amounts  (that  is,  18  to  20  ppm  of  dry 
soil;  sodium  acetate  extraction).  It  was 
further  noted  that  where  plants  did  not 
show  injury  some  were  rather  soft,  light 
in  color,  and  grew  quite  rapidly  (for 
example,  petunia)  ;  see  Section  14  for 
explanation  of  this  effect.  When  plants 
were  beyond  the  seedling  stage,  they 
apparentl)  were  unaffected  hy  this  con- 
dition,   (/rogan    and   /ink    (1956)    have 


also  recently  shown  that  free  ammonia 
or  ammonium  hydroxide  may  cause 
severe  injury  to  roots  and  tops  of  lettuce 
plants  in  California  fields.  Application 
of  organic  nitrogenous  fertilizers  to  cold 
waterlogged  soil,  or  the  use  of  aqua  or 
anhydrous  ammonia,  produced  the  in- 
jury. Ammonium  sulfate,  ammonium 
nitrate,  or  calcium  nitrate  caused  rela- 
tively little  damage. 


[112] 


Certain  bedding  plants  proved  to  be 
good  indicators  of  the  presence  of  am- 
monium. Phlox  and  verbena  suffered 
severe  leaf  burn;  carnations,  sweet  alys- 
sum,  and  stocks  lost  their  roots,  and 
cotyledons  turned  yellow.  Petunia,  on 
the  other  hand,  seemed  to  thrive  on 
moderate  amounts  of  ammonium,  but 
when  levels  became  too  high,  extreme 
iron  chlorosis  developed  in  the  youngest 
leaves.  Iron  chlorosis  was  also  noted  on 
snapdragon  seedlings. 

At  this  point  there  seemed  little  doubt 
that  the  problem  was  related  to  the 
organic  nitrogen  source  and  the  effect 
of  soil  treatment  on  the  organisms  in- 
volved in  nitrogen  conversion  (Sec.  14). 
The  following  explanation  may  enable 
the  grower  to  better  understand  the 
principles  involved. 

Organic  nitrogen  is  converted  to  the 
ammonium  form  in  several  steps  involv- 
ing the  activity  of  certain  microorgan- 
isms. This  group  consists  of  numerous 
fungi  and  bacteria  (including  spore 
formers) . 

The  conversion  of  ammonium  to 
nitrate  is  a  stepwise  process  involving 
heat-susceptible,  nonspore-forming  bac- 
teria, as  explained  in  Section  14.  Ob- 
viously, any  condition  which  affects  the 
population  or  the  activity  of  these  organ- 
isms will  affect  the  rate  of  nitrogen  re- 
lease from  the  organic,  and  conversion 
from  the  ammonium  form.  Treatment  of 
the  soil  with  steam  or  chemicals  to  rid  it 
of  pathogenic  fungi  and  bacteria  has  a 
marked  effect  on  the  population  of  these 
organisms  as  well  as  many  others  which 
may  be  present  (Sec.  14).  In  general, 
the  treatments  will  reduce  the  numbers 
of  the  organisms  which  produce  ammo- 
nium from  organic  nitrogen  and  will 
eradicate  bacteria  which  produce  nitrate 
from  ammonium.  These  facts,  which  have 
been  reported  by  many  workers,  were 
verified  in  the  following  experiments 
with  various  treatments  of  a  U.  C.-type 
mix,  followed  by  inoculation  with  am- 
monifying  and   nitrifying   organisms. 


Inoculation  tests 

In  order  to  determine  the  effect  of 
inoculation  on  nitrogen  release,  we  pre- 
pared a  soil  mix  of  the  U.  C.  type  using 
hoof  and  horn  (4%  lb.  per  cu.  yd.)  as 
the  source  of  organic  nitrogen,  without 
any  inorganic  nitrogen.  The  mix  was 
divided  into  six  groups: 

1.  Not  steamed,  not  inoculated; 

2.  Not  steamed,  inoculated; 

3.  Steamed  l/2  hour  at  212°  F,  not 
inoculated; 

4.  Steamed  %  hour  at  212°  F,  inocu- 
lated; 

5.  Steamed  8  hours  at  212°  F,  not 
inoculated; 

6.  Steamed  8  hours  at  212°  F,  inocu- 
lated. 

A  water  suspension  of  a  soil  known  to 
contain  ammonium  and  nitrate-produc- 
ing organisms  was  uniformly  mixed  with 
the  soil  for  series  2,  4,  and  6.  Series  1,  3, 
and  5  were  treated  with  an  equivalent 
volume  of  water  only.  All  containers  of 
soil  were  stored  at  70°  F.  Precautions 
were  taken  to  avoid  chance  contamina- 
tion, and  soils  were  not  allowed  to  dry 
and  thus  inhibit  organism  activity. 
Samples  of  soil  were  taken  at  the  start 
of  the  experiment  and  at  5,  7,  14,  21,  and 
28  days.  These  samples  were  immediately 
oven-dried  and  analyzed  for  available 
ammonium  and  nitrate  nitrogen.  A  1:5 
soil-water  suspension  was  shaken  with  an 
excess  of  calcium  sulfate  for  30  minutes 
before  filtering  and  analyzing. 

The  results  are  presented  graphically 
(fig.  68),  each  graph  representing  an 
average  of  three  replicates.  The  experi- 
ment gave  the  following  results: 

1.  When  reinoculation  was  prevented, 
steaming  completely  inhibited  ni- 
trate formation.  The  nitrifying  bac- 
teria were  completely  eradicated  by 
the  treatment. 

2.  Steaming  reduced  the  rate  of  am- 
monium production  but  did  not 
completely  inhibit  it.  Only  part  of 
the  ammonium  producers  were 
killed  by  steaming. 


[113] 


NOT  INOCULATED 


INOCULATED 


200" 


100- 


8        0 


- 

Not  stea 

med 
NH   + 

4 

N03        / 

• 

A 
s 

- 

—  -»_ 

I 

HOKy 

s 

1 

1 

Not  steamed 
NH.  +  NO 


200 


£ 

Q. 

Q. 


O) 

o 


0-1- 


Stea  med  Vi  hr. 


NH   +  NO 


Steamed  Vi  hr. 


NH    +  NO, 

4  •»■ 


NO3     / 

3  / 


,<r 


J>      200 

_g 

'5 
> 
< 


100- 


Steamed    8  hrs. 


NH   +  NO 


10 


20 


Time  (days) 


7o 


Steamed   8  hrs. 


NH4+NQ3 


NOa/ 


,-^ 


10 

Time  (days) 


20" 


7o 


Fig.  68.  Ammonium  and  nitrate  nitrogen  production  in  steamed  and  unsteamed  U.C.-type  soil        j 
mix,  with  and  without  inoculation   by  nitrogen-converting   organisms,   held  at  70°F.  The  figures 
represent  an  average  of  three  series.  See  p.   113  and  115  for  explanation. 


[  114] 


3.  There  was  no  significant  difference 
between  the  effect  of  V2"  and  3-hour 
steaming  on  organisms  which  pro- 
duce ammonium  and  nitrate  from 
the  organic  nitrogen.  The  treatment 
intervals  inhibited  activity  to  about 
the  same  degree. 

4.  Inoculation  reestablished  the  ni- 
trate-producing power  of  the  soil, 
but  nitrification  was  delayed  for 
about  10  days.  The  nitrate  produc- 
tion in  the  unsteamed  soil  was  also 
enhanced  by  inoculation,  indicating 
that  the  fine-sand  subsoil  used  was 
low  in  nitrifying  bacteria. 

5.  Inoculation  enhanced  ammonium 
production  in  all  cases,  but  did  so 
to  a  lesser  extent  in  the  unsteamed 
soils. 

In  the  nursery,  nitrate  is  produced  in 
a  steamed  soil  as  a  result  of  accidental 
inoculation.  Since  ammonifiers  are  both 
numerous  and  of  many  types  they  are 
better  able  to  reestablish  under  a  variety 
of  conditions  than  the  nitrifiers  (Sec. 
14).  If  inoculation  is  to  be  used  as  a 
means  of  reducing  ammonium  toxicity, 
it  will  be  desirable  to  exclude  ammo- 
nium-producing organisms  from  the 
inoculum.  Otherwise  the  enhanced  am- 
monium production  could  defeat  the 
purpose  of  the  inoculation. 

A  steamed  soil  will  normally  become 
reinoculated  with  nitrate-forming  bac- 
teria in  a  few  days  to  a  few  weeks.  When 
plants  are  sensitive  to  ammonium,  and 
conditions  are  favorable  for  its  accumu- 
lation, procedures  must  be  adopted  that 
will  avoid  damage.  At  present  the  pos- 
sibility of  artificial  inoculation  with 
nitrifying  bacteria  is  insufficiently  ex- 
plored to  be  practically  useful.  Presently 
the  only  other  approach  is  to  control  am- 
monium production  by  adjusting  the 
amount  and  type  of  organic  nitrogen 
used  in  relation  to  the  conditions  which 
affect  its  rate  of  release.  To  compare  the 
rate  of  breakdown  of  different  organic 
nitrogens  under  a  variety  of  treatment 


and   environmental   conditions,   another 
series  of  tests  was  run. 

Production  of  Available  Nitrogen 
from  Organic  Sources 

The  experiment  was  designed  to  de- 
termine the  effects  of  different  forms  of 
organic  nitrogen,  rate  of  addition,  steam- 
ing, and  storage  temperature  of  the  soil 
mix  upon  release  of  available  nitrogen. 
Organic  nitrogen  sources  were: 

1.  Castor  pomace  (5.75  per  cent  nitro- 
gen) ; 

2.  Cottonseed  meal   (7  per  cent  nitro- 
gen) ; 

3.  Fish  meal  (11  per  cent  nitrogen)  ; 

4.  Blood  meal  (13  per  cent  nitrogen)  ; 

5.  Hoof  and  horn  meal   (13  per  cent 
nitrogen)  ; 

6.  Urea-formaldehyde    resin    (35   per 
cent  nitrogen)  ; 

7.  Steer  manure  (2  per  cent  nitrogen) . 
Also  included  were : 

8.  Leaf  mold  in  place  of  peat  moss, 
without  other  nitrogen  source; 

9.  Control;    peat  moss,   but  no   other 
organic  nitrogen  source. 

Soil  mix  B  (25  per  cent  peat)  was 
used  in  all  except  no.  8  above.  Potassium, 
phosphate,  and  lime  were  added  to  all. 
Hoof  and  horn  was  added  at  the  rate  of 
4%  and  6%  pounds  per  cubic  yard;  the 
other  nitrogen  sources  were  added  to 
supply  the  same  amount  of  actual  nitro- 
gen. One  set  of  samples  of  each  form  of 
organic  nitrogen  was  steamed  (212°  F 
for  30  min.),  another  set  not  steamed, 
and  both  were  stored  at  50°  or  70°  F. 

Soils  were  protected  from  recon- 
tamination,  and  samples  taken  at  0,  11, 
24,  38,  52,  81,  102,  133,  and  149  days. 
Ammonium  and  nitrate  nitrogen  were 
determined  on  each  sample,  using  a  1:5 
soil-water  extraction  ratio  in  the  pres- 
ence of  excess  calcium  sulfate. 

Effect  of  treatments  on  release 
of  available  nitrogen 

Three  of  the  above  nine  series  (steer 
manure,    leaf    mold,    and    the    control) 


[115] 


c 
d) 
Ui 
O 


J) 
_Q 
_D 

'5 
> 

< 


STEAMED 


NH4+  NO 3 


— T— 

15 


50 
Time  (days) 


NOT  STEAMED 

NH4  +  N03    s"^*'* 
//'MO  , 

/                        I 

/                            1 

/                                  t 
/                                  / 
/                                  • 

1 1 

— i 

100 


15 


50 
Time  (days) 


100 


Fig.  69.  Diagrams  showing  general  character  of  the  effect  of  steaming  of  U.C.  mix  B  on  con- 
version of  organic  nitrogen  to  available  forms.  See  p.  115  and  116  for  explanation. 


produced  little  or  no  nitrogen ;  the  others 
produced  substantial  amounts.  The  check 
showed  little  available  nitrogen  under 
any  conditions;  toward  the  end  of  the 
experiment  there  was  a  slight  tendency 
for  available  nitrogen  to  increase,  prob- 
ably from  the  peat  moss,  which  normally 
contains  about  1  per  cent  nitrogen.  In 
the  mix  containing  leaf  mold,  the  avail- 
able nitrogen  level  was  higher  than  in 
the  control  but  was  affected  by  treatment 
only  when  the  unsteamed  mix  was 
stored  at  70°  F.  In  this  lot  an  appreciable 
amount  of  available  nitrogen  accumu- 
lated during  five  months'  storage,  but 
the  amount  produced  did  not  compare 
with  that  from  nitrogen  sources  1 
through  6.  Steer  manure  showed  some 
active  release  of  nitrogen  at  70°  after 
50  days  or  more.  The  amount  was  less 
than  that  released  from  leaf  mold,  how- 
ever, and  is  considered  to  be  of  little 
consequence  from  a  practical  standpoint. 
Initial  tests  indicated  that  the  manure 
mix  was  very  saline. 

Organic  nitrogen   forms  1  through  6 


were  sufficiently  similar  in  their  pattern 
of  breakdown  to  be  discussed  together. 
Two  diagrams  (fig.  69)  show  this 
general  pattern.  Whether  steamed  or  not, 
nitrogen  was  more  rapidly  released  dur- 
ing the  first  month  than  in  the  next  two 
months.  When  the  mixtures  were  un- 
steamed, nitrate  was  produced;  but  there 
was  a  delay  before  the  rate  was  sufficient 
to  prevent  ammonium  accumulation. 
When  the  mixtures  were  steamed,  no 
nitrate  was  produced  in  100  days,  in- 
dicating that  reinoculation  by  nitrifying 
bacteria  had  not  occurred. 

In  unsteamed  soil  the  time  interval 
before  nitrification  began  was  greatly  in- 
creased by  low  temperature  and  by  low 
organic  nitrogen.  The  results  are  sum- 
marized in  the  tabulation  below. 
One  may  conclude  that  these  condi- 
tions retarded  the  activity  of  nitrifying 
organisms. 

Further  indication  of  the  effect  of 
these  factors  upon  rate  of  nitrogen  re- 
lease is  shown  by  the  average  time  re- 
quired for  available  nitrogen  to  reach  50 


Storage  temperature 

Time  required  for  nitrification  to  begin  (unsteamed  soil) 

With  4'j  lb.  rate 

With  B\-i  lb.  rate 

50    F 

70    F 

60-130  days,  av.  95  days 
10-40  days,  av.  30  days 

60-110  days,  av.  88  days 
10-25  days,  av.  18  days 

[116] 


Storage  temperature 

Soil 
treatment 

Average  storage  time  to  reach  50  ppm 
of  available  nitrogen 

With  VAVa.  rate 

With  SH  lb.  rate 

50°  F 

f  Steamed 
\  Unsteamed 
f  Steamed 
\  Unsteamed 

37  days 
27  days 
18  days 
16  days 

28  days 
19  days 
12  days 
10  days 

70°  F 

ppm  of  dry  soil,  a  moderate  supply  for 
plant  growth  in  soil  mix  B;  the  data  are 
summarized  above. 

An  increase  in  the  storage  temperature 
resulted  in  an  increase  in  the  rate  of 
available-nitrogen  production.  Increas- 
ing the  amount  of  source  material  also 
increased  this  rate.  On  the  other  hand, 
steaming  had  a  retarding  effect.  The  rate 
of  release  of  available  nitrogen  was  re- 
duced by  steaming  the  soil,  by  low 
storage  temperature,  and  by  low  rate  of 
addition  of  organic  nitrogen.  These  con- 
ditions evidently  retarded  the  activity  of 
organisms  producing  ammonium  nitro- 
gen from  organic  sources. 

At  the  time  of  the  first  sampling  (11 
days)  there  was,  in  this  expsriment,  an 
appreciable  increase  in  the  ammonium 
concentration  in  all  but  the  steamed  soils 


stored  at  50°  F.  In  these,  ammonium 
production  was  only  slight,  except  for 
urea-formaldehyde,  to  be  discussed 
later.  In  the  previously  described  (fig. 
68)  experiments  on  inoculation,  ammo- 
nium production  was  appreciable  at  5 
days.  It  seems  that  if  the  temperature  is 
high  enough,  activity  of  ammonifiers  be- 
gins very  quickly,  certainly  within  a  few 
days.  Still  another  indication  of  the  in- 
fluence of  these  environmental  conditions 
on  rate  of  nitrogen  release  is  shown  by 
the  concentration  of  available  nitrogen 
at  given  times  in  the  soil  mix;  the  data 
are  given  in  table  11. 

Again,  the  rate  of  release  of  available 
nitrogen  was  reduced  by  steaming,  low 
storage  temperatures,  and  low  rate  of 
nitrogen  addition.  In  all  cases  the 
amount    of    available    nitrogen    evolved 


Table  1 1 .  Effect  of  Storage  Interval  on  Concentration  of  Available 

Nitrogen  in  Soil  Mix 
Average  of  six  organic  nitrogen  sources 

Storage  temperature,  °F 

Treatment 

Concentration  of  available  nitrogen,*  in  ppm  of  dry  soil 

With  \y2  lb.  ratef 

With  6%  lb.  ratet 

15 
days 

50 
days 

100 
days 

15 
days 

50 

days 

100 
days 

50 

f  Steamed 
(Unsteamed 

{ Steamed 
\  Unsteamed 

20 
33 

45 
61 

74 
86 

80 
120 

80 
96 

95 
159 

33 

52 

63 
83 

88 
94 

108 
180 

96 
126 

108 
214 

70 

*  Average  of  total  available  (ammonium  plus  nitrate)  nitrogen  from  organic  nitrogen  sources  1  through  6. 
t  Hoof  and  horn  meal  added  at  4'2  and  612  pounds  per  cubic  yard.  Other  nitrogen  sources  were  added 
to  supply  an  equivalent  amount  of  nitrogen. 

[117] 


was  substantially  greater  in  the  first  than 
in  the  second  50-day  period.  This  ten- 
dency for  leveling-off  was  greater  in 
steamed  than  in  unsteamed  soil,  as 
shown  in  figure  69. 

Nitrogen  supplied  by 
specific  materials 

There  were  marked  differences  be- 
tween the  available  nitrogen  released 
from  the  six  different  organic  sources 
(table  12). 

The  materials  (castor  pomace,  cotton- 
seed meal,  and  fish  meal)  that  most 
rapidly  produced  available  nitrogen, 
were,  surprisingly  enough,  those  with  the 
lowest  original  nitrogen  content.  Of  the 
three,  castor  pomace  seemed  to  break 
down  most  rapidly.  Blood  meal,  hoof 
and  horn  meal,  and  urea-formaldehyde 
all  have  a  much  lower  rate  of  release  of 
available  nitrogen,  and  the  values  are 
considerably  lower  than  those  of  the  first 
three. 

Steaming  enhances  the  breakdown  of 


urea-formaldehyde,  whereas  the  effect 
was  just  the  opposite  in  all  other  mate- 
rials. In  the  more  complete  data  it  is 
apparent  that  there  was  an  immediate 
build-up  of  ammonium  whenever  the 
urea-formaldehyde  mix  was  steamed. 
Possibly  the  effect  of  steaming  was  to 
partially  hydrolyze  the  compound  to  pro- 
duce some  free  urea,  which  was  quickly 
converted  to  ammonium.  After  this  first 
surge,  the  rate  of  nitrogen  release  was 
very  slow.  In  all  cases  where  the  mixes 
were  not  steamed,  the  rate  of  release  was 
slower  from  urea-formaldehyde  than 
from  any  of  the  others.  Thus,  urea-for- 
maldehyde has  certain  desirable  prop- 
erties, but  a  serious  drawback  of  initially 
releasing  ammonium  when  steamed. 

As  a  result  of  these  tests  the  materials 
might  be  listed  in  the  descending  order 
of  their  activity  as  follows:  castor 
pomace;  fish  meal;  cottonseed  meal; 
blood  meal  and  hoof  and  horn  (equal)  ; 
urea-formaldehyde. 

This  information  does  not  necessarily 


Table  12.  Available  Nitrogen  Released  by  Various  Organic  Nitrogen 

Sources  in  a  Soil  Mix  After  30  Days'  Storage 
Two  rates  of  organic  application,  steamed  and  unsteamed,  and  stored  at  two 

temperatures 

Storage 
temperature, 

op 

Treatment 

Rate, 

lb.  per 

cu.  yd.f 

Concentration  of  available  nitrogen,*  in  ppm 

Castor 
pomace 

Cotton- 
seed 
meal 

Fish 
meal 

Blood 
meal 

Hoof 
and 
horn 
meal 

Urea- 
formal- 
dehyde 

resin 

50 

< 

Steamed 
Unsteamed 
Steamed 
Unsteamed 

UA 

Wa 
Wa 
Wa 
Wa 
Wa 
Wa 
Wa 

60 
100 

93 
128 

73 
132 
160 
160 

54 

75 

108 

116 

60 

98 

95 

140 

60 

92 

80 

130 

66 

114 

104 

160 

25 
48 
47 
60 
45 
60 
63 
105 

25 
37 
47 
63 
50 
60 
70 
95 

80 
73 
42 
46 
78 
64 
60 
64 

70 

Average  steame 

d 

91 
130 

72 

115 

83 
119 

45 
69 

43 
69 

74 
53 

Average  unstea] 

ned. 

*  Total  available  nitrogen  (ammonium  plus  nitrate)  in  ppm  of  dry  soil.  In  steamed  (212°  F  for  30  min.) 
series  all  nitrogen  was  ammonium. 

t  Nitrogen  addition  equivalent  to  41  £  lb.  or  61  jj  lb.  hoof  and  horn  meal  per  cu.  yd. 

hold  when  these  fertilizers  are  applied 
broadcast  or  as  top  dressings  (see  "Sur- 
face Application  versus  Mixing,"  below) . 

Hoof  and  horn  meal  and  blood  meal 
constitute  the  most  desirable  sources  of 
organic  nitrogen  for  the  soil  mix,  in  that 
they  are  slower  in  their  release  of  avail- 
able nitrogen  than  are  castor  pomace, 
cottonseed  meal,  and  fish  meal.  If  the 
problem  of  initial  ammonium  release 
upon  steaming  can  be  overcome  or  mini- 
mized, urea-formaldehyde  might  be 
superior  to  any  of  the  natural  sources — 
assuming,  of  course,  that  biuret  toxicity 
can  be  avoided  (Sec.  5). 

Until  further  investigations  are  carried 
out,  the  effects  of  thorough  chemical 
treatment  must  be  assumed  to  produce 
results  similar  to  those  found  where 
steaming  was  used.  Since  heat  is  prob- 
ably a  major  factor  in  the  hydrolysis  of 
the  urea-formaldehyde,  chemical  treat- 
ment might  prevent  the  rapid  breakdown 
to  ammonium  by  this  material. 

Application  of  This  Information 

Bedding  plants 

Identification  and  clarification  of  the 
problems  discussed  above  led  to  certain 
changes  in  bedding-plant  procedure.  The 
use  of  organic  nitrogen  in  the  mixes  was 
greatly  reduced  or  eliminated.  This  re- 
duction in  nitrogen  supply  was  offset  by 
the  use  of  nitrate  forms.  Calcium  nitrate 
solution  was  used  as  a  liquid  supplement 
and  in  many  instances  potassium  nitrate 
has  been  substituted  for  potassium  sul- 
fate in  the  mix.  In  addition,  hoof  and 
horn  or  blood  meal  was  applied  as  a  top 
dressing,  this  procedure  supplying 
slowly  available  nitrogen  over  an  ex- 
tended period  without  the  dangers  of 
ammonium  toxicity  (see  "Surface  Appli- 
cation versus  Mixing,"  below).  It  was 
especially  important  to  avoid  large 
amounts  of  organic  nitrogen  in  the  mix 
during  periods  of  hot  weather. 


Pot  plants 

While  the  work  with  bedding-plant 
growers  was  progressing,  attention  was 
given  to  similar  problems  in  greenhouse- 
pot  and  nursery-can  culture.  In  pot-plant 
growing  the  only  period  of  high  sen- 
sitivity seemed  to  be  in  the  early  growth 
of  rooted  cuttings  or  small  seedlings. 
Where  plants  were  vigorous,  there  was 
seldom  any  setback  if  reasonable 
amounts  of  organic  nitrogen  were  used. 
No  problems  developed  where  plants 
were  moved  up  from  a  smaller  to  a  larger 
container.  There  are  two  primary 
reasons  for  the  increased  safety  in  "pot- 
ting-on": 

1.  The  amount  of  soil  used  for  each 
plant  is  substantially  less  in  proportion 
to  plant  size  than  when  potting  seedlings 
or  rooted  cuttings  with  no  root  ball.  The 
established  plant  will  require  much  more 
nitrogen  and  must  get  it  from  a  relatively 
smaller  amount  of  added  soil. 

2.  The  root  ball  which  is  placed  in  the 
new  soil  has  had  time  to  become  reinocu- 
lated  with  nitrifying  bacteria.  The  added 
soil  will,  therefore,  become  quickly 
inoculated  and  nitrate  will  be  produced. 

In  some  cases,  growers  are  practicing 
subirrigation  for  such  crops  as  Saint- 
paulia.  In  this  procedure  there  is  almost 
no  leaching  action  and  the  added  nitro- 
gen is  not  so  readily  lost  as  with  surface 
watering.  Under  these  conditions  the 
upper  limit  of  added  hoof  and  horn  has 
been  about  2%  pounds  per  cubic  yard  of 
soil. 

Nursery-can  culture 

In  nursery-can  growing  the  problems 
are  similar  to  those  encountered  in  the 
bedding-plant  industry.  Rooted  cuttings 
or  small  liners  are  transplanted  into  gal- 
lon cans  where  growth  of  many  items 
may  be  relatively  slow.  This  provides 
ample  opportunity  for  added  organic 
nitrogen  to  supply  an  excess  of  available 
nitrogen.  Although  no  indications  of 
specific  toxicity  due  to  ammonium  have 


[119] 


been  noticed,  there  have  been  instances 
where  the  build-up  of  nitrogen  was  suf- 
ficient to  produce  a  salinity  effect.  For 
this  reason  organic  nitrogen  is  com- 
monly omitted  or  used  in  relatively  small 
amounts.  Another  reason  for  reducing 
the  organic  nitrogen  is  that  soils  are  fre- 
quently prepared  ahead  of  time  and  held 
for  several  weeks  to  several  months  be- 
fore planting  (Sec.  5). 


Bed  or  bench  culture 

When  crops  are  grown  for  several 
seasons  or  years  in  beds  or  benches,  or- 
ganic nitrogen  may  be  completely 
omitted  in  the  mix  and  a  program  of 
fertilizing,  with  or  without  organic  nitro- 
gen, followed  from  the  start.  There  is 
little  advantage  in  delaying  the  initiation 
of  the  long-term  program  by  initial  mix- 
ing of  organic  nitrogen  into  the  soil. 


FACTORS  AFFECTING  NITROGEN  RELEASE 
FROM  ORGANIC  SOURCES 


In  well-aerated  soils  such  as  those  ob- 
tained in  the  physical  medium  of  the 
U.  C.-type  mix,  the  following  factors  may 
affect  the  amount  and  rate  of  release  of 
available  nitrogen  from  the  organic 
form. 

Microorganism  population 

In  natural  soils  the  population  of 
microorganisms  will  be  much  higher  in 
the  surface  6  to  12  inches  than  at  greater 
depths.  Subsoils  are  frequently  low  in 
organism  population.  This  is  to  be  ex- 
pected since  the  moisture,  air,  and  or- 
ganic matter  necessary  for  most  organ- 
ism activity  are  more  available  in  surface 
soils.  The  difference  in  the  effect  on  rate 
of  nitrogen  release  by  untreated  surface 
and  subsoils  may  be  substantial. 

Where  heat  or  chemical  treatment  is 
practiced,  the  difference  between  surface 
and  subsoils  is  reduced,  since  all  nitrify- 
ing bacteria  are  killed  and  the  population 
of  those  which  produce  ammonium  is 
initially  reduced   (Sec.  14). 

Heat  and  chemical  treatment 

As  suggested  in  the  preceding  para- 
graph, the  procedure  of  treating  the  soil 
to  rid  it  of  pathogens  necessarily  affects 
the  whole  microorganism  population. 
Nitrifying  bacteria  will  be  eradicated 
where  treatmenl  is  effective.  Ammonium 
production  may  be  reduced  but  not  en- 


tirely eliminated.  Furthermore,  ammo- 
nifiers  will  more  rapidly  repopulate  the 
soil.  Thus  ammonium  may  be  produced, 
but  not  converted  to  nitrate,  until  the  soil 
becomes  reinoculated  with  nitrifiers. 
Under  normal  conditions,  inoculation 
will  occur  within  a  period  of  several  days 
to  several  weeks.  This  is  an  uncertainty 
which  in  the  future  may  be  eliminated 
by  the  use  of  inoculation  cultures. 

Soil  reaction  (pH) 

Highly  acid  media  generally  have  little 
effect  on  the  activity  of  ammonium  pro- 
ducers, while  inhibiting  the  activity  of 
nitrifiers  (Sec.  14).  This  was  shown  in 
a  nursery  test  in  which  azaleas  were 
grown  in  beds  of  pure  peat  plus  organic 
nitrogen.  Hydrated  lime  was  worked  into 
a  test  bed  before  planting.  After  several 
months,  plants  in  the  limed  area  were 
noted  to  be  darker  in  color  than  the  re- 
mainder. Tests  of  the  growing  medium 
showed  the  differences  in  acidity  and 
nitrogen  concentration  as  tabulated  on 
page  121. 

It  has  been  demonstrated  by  Tied j  ens 
and  Robbins  (1931)  that  some  crop 
plants  are  unable  to  utilize  the  ammo- 
nium form  of  nitrogen  when  the  pH  is 
low,  but  do  so  readily  when  the  pH  is  in 
the  neutral  to  alkaline  range.  The  use  of 
lime  in  recommended  mixes  should  de- 
crease difficulties  of  this  nature. 


[120] 


Test  bed 

PH 

Nitrate  nitrogen, 
ppm  of  dry  peat 

Ammonium  nitro- 
gen, ppm  of 
dry  peat 

Limed 

Nonlimed 

5.9 
3.9 

350 
26 

155 
225 

Soil  temperature 

In  general,  the  warmer  the  soil,  the 
greater  will  be  the  activity  of  micro- 
organisms in  it.  There  is  an  upper  limit, 
of  course,  beyond  which  an  increase  in 
temperature  will  reduce  and  even  kill 
them.  When  temperatures  are  low,  ac- 
tivity is  reduced.  The  nitrifiers  are  more 
critically  affected  than  are  the  ammoni- 
fiers  (Sec.  14) .  Thus  during  cool  weather 
there  will  be  a  reduced  rate  of  ammo- 
nium production,  and  an  even  greater 
reduction  in  the  rate  of  nitrification. 
When  soil  temperatures  are  below  40°  F, 
applications  of  blood  meal  or  other 
organic  sources  may  be  quite  ineffective 
and  the  rate  of  release  of  nitrogen  too 
slow  to  meet  the  plant  requirements. 
Under  these  conditions  the  grower 
should  use  soluble  materials  such  as 
calcium  nitrate. 

Concentration  and  source 
of  organic  nitrogen 

As  already  shown,  an  increase  in 
amount  of  organic  nitrogen  added  to  a 
soil  mix  will  increase  the  rate  of  release 
as  well  as  the  total  amount  of  available 
nitrogen.  Therefore,  the  higher  rate  of 
addition  does  not  necessarily  mean  that 
the  period  of  release  will  be  lengthened. 

The  sources  of  organic  nitrogen  are 
quite  varied  in  chemical  composition. 
Some  organic  sources  of  nitrogen  seem 
to  be  more  readily  assimilated  by  organ- 
isms which  decompose  them  than  do 
others.  Urea- formaldehyde,  hoof  and 
horn  meal,  and  blood  meal  are  among 
the  slowly  decomposable  forms  of  or- 
ganic nitrogen.  Cottonseed  meal,  castor 
pomace,  and  fish  meal  are  quite  rapidly 
decomposed.  Since  the  primary  objective 
in  adding  an  organic  nitrogen  source  to 


the  mix  is  one  of  prolonging  the  period 
of  release  of  available  nitrogen,  it  is  most 
reasonable  to  add  those  sources  which 
are  slowest  in  rate  of  breakdown. 

As  explained  below,  there  are  substan- 
tial differences  in  rate  of  release  of  avail- 
able nitrogen  when  organic  materials 
are  broadcast  over  the  surface  of  the  soil 
as  compared  with  being  mixed  into  it. 
Presumably  the  more  finely  divided  and 
more  "palatable"  materials  would  result 
in  more  rapid  release  of  available  nitro- 
gen with  surface  application. 

Moisture  and  aeration 

A  soil  that  is  dry  will  not  support 
plant  growth  and,  as  might  be  expected, 
will  retard  microorganism  activity.  It  is, 
however,  unsafe  to  assume  that  a  stored 
soil  mix  is  dry  enough  to  prevent  the  re- 
lease of  available  nitrogen  from  organic 
sources.  In  some  cases  oven-dried  and 
stored  samples  have  been  found  to  pro- 
duce available  nitrogen.  Lack  of  ade- 
quate moisture  would  probably  be  more 
damaging  to  the  growing  plant  than  any 
side-effect  it  might  have  on  nitrogen 
relations. 

Lack  of  oxygen  supply  (aeration)  is 
an  inhibiting  factor  in  plant  growth  and 
alters  microorganism  activity.  It  has 
commonly  been  accepted  that  poor  aera- 
tion will  result  in  nitrite  accumulation 
(Sec.  14) .  It  now  seems  that  many  of  the 
troubles  blamed  on  nitrite  may  have 
been  due  to  ammonium  toxicity  and 
salinity.  There  is  recent  evidence  ( Duis- 
berg  and  Buehrer,  1954)  that  nitrite 
toxicity  may  have  been  highly  overrated. 
In  any  event,  a  U.  C.-type  soil  mix  pro- 
vides a  medium  with  excellent  aeration, 
and  chemically  stable  to  soil  treatment. 
Nitrite  accumulation  has  thus  far  not 
been  found  to  occur  in  it. 


[121] 


SURFACE  APPLICATION  VERSUS  MIXING 


Surface  application  of  organic  nitro- 
gen can  be  substantially  less  dangerous 
from  the  ammonium-toxicity  standpoint 
than  when  it  is  mixed  into  the  soil  mass. 
This  greater  safety  results  from: 

1.  The  slower  production  of  available 
nitrogen  at  the  surface,  owing  to 
less  intimate  contact  with  the  soil 
mass  and  to  environmental  condi- 
tions intermittently  unfavorable  to 
soil  organisms. 

2.  The  restricted  ability  of  ammonium 
to  move  through  the  soil  into  the 
root  zone  (see  "Ammonium  nitro- 
gen," above) . 

3.  More  rapid  inoculation  with  nitri- 
fiers  at  the  surface  of  treated  soil, 
converting  to  nitrate  the  ammonium 
produced.  Since  top-dressing  mate- 
rials are  usually  not  treated  with 
heat  or  chemicals,  they  may  have  a 
better  balance  between  ammonify- 
ing and  nitrifying  microorganisms. 


The  restrictions  on  the  use  of  organic 
nitrogen  in  the  soil  mass  discussed  in  this 
section  do  not  necessarily  apply  to  their 
use  as  surface  dressings. 

Features  of  surface  application  that 
are  less  desirable  than  soil-mix  inclusion 
are: 

1.  Practical  inability  to  make  uniform 
application ; 

2.  Danger  of  plant  damage  from  rapid 
decomposition  at  high  temperatures 
in  contact  with  the  seedling ; 

3.  Objectionable  residues,  odors,  and 
flies. 

Under  some  circumstances  it  is  better 
to  mix  the  organic  nitrogen  into  the  soil 
than  to  apply  it  as  a  later  top  dressing. 
The  dangers  discussed  in  this  section 
from  mixing  organic  nitrogen  into  the 
soil  mass  encourage  cautious  and  judi- 
cious use  of  it  in  this  way.  On  the  other 
hand,  there  are  also  problems  in  using 
the  materials  in  surface  application. 


ADJUSTING  TO  SPECIFIC  SITUATIONS 


A  grower  is  never  relieved  of  the 
necessity  of  thinking  by  this  or  any  other 
system  of  soils  and  fertilizers.  The  fore- 
going discussion  illustrates  the  complex- 
ities of  nitrogen  application  in  plant 
growing.  The  problem  of  ammonium 
toxicity  is  an  important  consideration 
where  organic  forms  are  used,  but  they 
may  greatly  prolong  the  usefulness  of  the 
soil  mix  and  prevent  the  occurrence  of 
extreme  nitrogen  deficiency.  In  some 
growing   operations    (for   example,   pot 


plants)  organic  nitrogen  included  in  the 
mix  reduces  labor  and  variability  from 
top-dressing  application.  Where  the  U.  C. 
system  is  carefully  followed  the  only 
nutritional  problems  which  can  develop 
in  the  first  several  months  of  use  must  be 
related  to  nitrogen  supply.  It  is  im- 
portant whether  nitrogen  is  supplied  as 
ammonium  or  nitrate,  and  in  what  quan- 
tity. The  information  in  this  section 
should  aid  in  crop  programming  and 
analysis  of  nitrogen  nutrition  problems. 


r  122  ] 


SECTION 


Heat  Treatment 
of  Soil 


Kenneth  F.  Baker 
Chester  N.  Roistacher 


Comparison  of  commonly  used  treatments 

Benefits  from  heat  treatment  of  soil 

Sanitary  precautions  in  soil  treatments 

Treatment  temperature  and  time 

The  form  of  steam  used 

Efficient  soil  steaming 

Volume  of  steam  required 

Preparing  soil  for  steaming 

Uneven  heating 

Cooling  the  treated  soil 

Water  content  after  steaming 

Steam-treating  home  yards 

Cost  of  steaming  soil 


.HE  soil  AND  the  host  plant  are  the  two 
ultimate  sources  of  organisms  that  cause 
plant  disease.  Pathogens  may  be  eradi- 
cated from  the  soil  by  treatment  with 
heat  (this  section  and  sees.  9  and  10) 
or  chemicals  (Sec.  11),  and  from  the 
planting  stock  (Sec.  13)   and  containers 


(Sec.  12)  in  various  ways.  These  prac- 
tices, along  with  the  use  of  a  U.  C.-type 
soil  mix  (sees.  5,  6,  and  7)  and  sanitary 
practices  (sees.  1,  3,  and  14).  are  the 
supports  for  economic  modern  plant 
production  through  the  U.  C.  system 
(frontispiece). 


COMPARISON  OF  COMMONLY  USED  TREATMENTS 


Heat  treatment  of  soil  may  be  done 
with  steam,  a  dry  source  of  heat,  or  hot 
water.  Each  has  its  place  in  nursery  prac- 
tice, the  same  as  chemical  treatment. 


Steam  versus  chemicals 

The  comparative  advantages  of  steam 
and  the  two  most  commonly  used  chemi- 
cals are  shown  in  table   13.  Treatment 


[123] 


Table  1 3.  Comparative  Advantages  of  Steam  and  Chemical  Treatments 
of  Soil  in  Common  Use  in  California  Nurseries 

Characteristic 

Steam,  180°-212°  F 
for  30  min. 

Methyl  bromide, 
4  lb.  per  100  cu.  ft. 

Chloropicrin, 
5  cc  per  cu.  ft. 

Time  required  for 
treatment 

About  1  hr. 

24-48  hr. 

2-3  days 

Time  between  treat- 
ment and  planting  .  . 

About  1-2  hr.  to  cool 

24-48  hr. 

7-10  days 

Kills  all  pathogens, 
weeds,  and  insects?. 

Yes,  best  treatment; 
a  few  weeds  sur- 
vive 

Most,  but  not  Ver- 
ticillium;   a    few 
weeds   survive 

Yes ;  a  few  weeds 
survive 

When  can  penetration 
of  material  be  deter- 
mined, as  a  measure 
of  effectiveness? .... 

At  once,  by  measur- 
ing soil  tempera- 
ture 

Later,  by  noting  re- 
duction of  disease 
or  pathogen 

Later,  by  noting  re- 
duction of  disease 
or  pathogen 

Toxic  after-effect  to 
crops? 

None  with  U.C.- 
type  soil  mixes 

Yes,  for  carnations 
and  some  others 

None,  when  properly 
aerated 

Use  near  living  plants? 

Yes 

Within  3  ft.  if  ade- 
quately ventilated 

Only  with  excellent 
ventilation 

Destroys  organisms  in 
unrotted  crop  refuse? 

Yes 

Yes 

Poorly 

Can  it  be  used  any- 
where?   

Only  if  portable 
boiler  used 

Yes 

Yes 

Is  its  use  limited  by 
environment? 

Time  and  cost  in- 
crease with  cold 
or  wet  soil,  but 
can  be  so  used 

Not  recommended 
below  60°  F 

Dosage  increased  if 
soil  below  65°  F 
or  wet 

Ease  of  application .... 

Easy 

Easy 

Obnoxious  and  time 
consuming 

Dangerous  to  work- 
men?   

No 

Yes 

Yes 

Is  large  capital  outlay 
required? 

If  boiler  unavailable 

No 

No 

Cost  per  cu.  ft.  of  soil, 
exclusive  of  labor  .  . 

Less  than  2  cents, 
including  equip- 
ment cost 

About  2.9-3.2  cents, 
excluding  equip- 
ment cost 

About  1.9-3.0  cents, 
excluding  equip- 
ment cost 

[124] 


with  steam  is  faster,  easier,  cheaper,  and 
more  effective  than  with  these  materials. 
It  remains  the  best  general  method  of 
disinfesting  soil,  destroying  fungi,  bac- 
teria, nematodes,  weeds,  and  insects.  It 
is  the  standard  for  judging  new  chemical 
treatments.  For  these  reasons  it  is  the 
treatment  emphasized  in  this  section. 

Steam  offers  further  advantages.  It  is 
more  dependable  and  its  effectiveness  is 
more  readily  determined  than  most 
chemical  treatments.  The  penetration  of 
steam  can  be  quickly  and  easily  assessed 
by  measuring  temperature  and  time.  This 
is  so  dependable  a  measure  of  effective- 
ness that  temperature  is  practically  used 
for  its  evaluation,  instead  of  the  measur- 
ing of  disease  as  must  be  done  with 
chemicals.  Treatment  by  steam  is  funda- 
mentally a  transfer  of  heat  from  the 
boiler  to  the  soil,  and  all  factors  (such 
as  soil  moisture,  porosity,  volume,  and 
temperature)  affect  a  single  variable,  the 
temperature,  measured  by  a  thermom- 
eter. The  effectiveness  of  chemicals  is 
modified  by  many  of  the  same  factors, 
but  the  result  (concentration  of  active 
chemical  in  the  soil)  is  not  readily  de- 
termined. Thus,  it  is  commercial  practice 
to  judge  penetration  of  a  chemical  by  the 
control  of  pathogens.  Practically,  this 
measurement  is  only  possible  weeks  or 
months  later,  and  by  the  occurrence  of 
disease. 

Soil  may  be  steamed,  furthermore, 
within  1  or  2  feet  of  living  plants  without 
injury  to  them.  This  is  a  distinct  advan- 
tage in  a  planted  glasshouse,  where  it 
may  be  necessary  to  treat  a  single  bench 
or  a  localized  area  of  it.  Chemicals  can 
be  used  in  this  way  only  with  excellent 
ventilation  and  even  then  there  is  some 
risk  (Sec.  11) . 

There  is  no  hazard  from  steaming  soil 
in  the  headhouse  room  or  other  places 
where  men  are  working.  In  closed  areas 
the  heat  may  be  uncomfortable,  but  no 
noxious  or  dangerous  gases  are  given  off. 
There  have  been  no  complaints  from 
neighbors   when   steam   has   been   used, 


whereas  serious  difficulties  have  some- 
times arisen  from  chemical  fumigation. 
There  are,  however,  some  conditions 
where  steam  is  impractical  (for  example, 
large  field  areas  used  for  low-value 
crops),  and  some  where  it  is  initially 
expensive  (for  example,  if  a  boiler  must 
be  purchased).  In  such  cases  chemical 
treatments  are  often  used  (Sec.  11). 
Some  chemicals  are  occasionally  injected 
with  steam  (Sec.  10,  type  26) ,  and  others 
are  applied  as  supplements  to  it   (Sec. 

id. 

Steam  versus 

other  heat  treatments 

A  dry  source  of  heat  (for  example, 
metal  heated  by  a  flame  or  electricity) 
may  be  used  for  treating  soil.  One  of  the 
worst  disadvantages  of  this  type  of  heat 
is  that  it  is  necessary  to  apply  intense 
heat  (high  temperature)  in  a  limited 
area  in  order  to  impart  the  required 
quantity  (B.t.u.).  Usually  intensity  is 
high,  quantity  is  small,  and  distribution 
through  the  soil  is  poor.  Steam,  in  con- 
trast, imparts  a  large  quantity  of  heat  at 
low  intensity  (212°  F),  and  flows 
through  the  soil  to  the  cold  area.  It  is 
almost  as  though  successive  heat  sources 
were  turned  on  along  the  advancing 
front  as  the  steam  moves  forward.  One 
of  the  principal  advantages  of  steam  is 
that  the  B.t.u.  are  released  at  the  point 
to  be  heated.  Furthermore,  there  is  a 
natural  ceiling  of  212°  when  soil  is 
heated  by  flowing  steam.  This  is  a  safety 
feature  shared  by  no  other  heat  treat- 
ment except  by  hot  water. 

From  these  facts  it  is  concluded  that 
a  dry  source  of  heat  may  be  used  for 
treating  soil  that  moves  past  the  heater. 
When  handled  in  this  way,  satisfactorily 
uniform  progressive  heating  of  the  whole 
soil  mass  results.  It  should  not  be  used 
to  treat  a  stationary  soil  mass.  In  con- 
trast, steam  is  most  efficiently  used  for 
treating  a  stationary  soil  mass.  When  a 
dry  source  of  heat  is  used  with  a  moving 
soil  mass,  it  is  necessary  to  treat  con- 


[125] 


tainers  in  some  other  way  (sees.  10  and 
12). 

The  principal  disadvantage  of  hot 
water,  even  when  boiling  (212°  F).  is 
that  it  releases  so  much  less  heat  per 
pound  to  the  soil  than  does  steam  (Sec. 
9) .  Hence  much  more  of  it  must  be  used 
to  raise  the  soil  temperature  to  the  same 
level.  Soil  may  be  puddled  by  such  treat- 
ment, and  the  quantity  of  water  draining 
from  the  soil  is  messy  and  troublesome, 


particularly  in  glasshouse  beds.  Hot 
water  is  less  efficient  and  convenient 
than  steam  for  treating  soil.  About  the 
only  compensating  advantage  is  that 
salinity  is  reduced  to  a  low  level  by  the 
leaching  provided.  Hot  water  is  some- 
times used  for  treating  propagating 
sand  (Sec.  10,  type  25).  It  may  be  used 
for  leaching  of  salts  from  propagating 
beds,  which  may  then  be  steamed.  Its  use 
is  decreasing. 


BENEFITS  FROM  HEAT  TREATMENT  OF  SOIL 


The  primary  reason  for  most  soil  heat 
treatments  is  the  elimination  of  fungi, 
bacteria,  and  nematodes  that  cause  plant 
disease.  There  are,  however,  other 
benefits. 

Heavy  soils  become  more  granular, 
with  improvement  of  drainage  and  aera- 
tion. Much  of  the  steaming  of  glasshouse 
rose  soils  in  the  United  States  is  done  for 
this  purpose,  rather  than  for  disease  con- 
trol. This  same  effect,  however,  often 
causes  trouble  for  bedding-plant  growers 
when  they  begin  steaming  and  do  not 
properly  adjust  watering  operations, 
since  the  stock  may  not  be  kept  suffi- 
ciently moist.  The  formation  of  toxins 
from  steaming  such  heavy  soils  is  dis- 
cussed in  Section  6. 

Improvement  of  plant  growth  not 
definitely  associated  with  elimination  of 
known  disease  sometimes  occurs.  This 
may  result  from  increased  availability  of 
nutrients  (Sec.  6),  change  from  nitrate 
to  ammonium  nutrition  (Sec.  7),  or  im- 


proved physical  structure  of  the  soil,  but 
is  often  due  to  a  biological  change  that 
may  or  may  not  be  directly  associated 
with  disease.  This  corresponds  to  the 
"increased  growth  response"  from  chem- 
ical treatment  of  soil  (fig.  119;  and  sees. 
11  and  14). 

Elimination  of  weeds  is  the  benefit 
from  heat  or  chemical  treatment  of  soil 
over  which  many  growers  are  at  first 
most  enthusiastic,  perhaps  because  of  the 
spectacular  results.  Many  bedding-plant 
growers  have  stated  that  this  feature 
alone  pays  for  the  treatment.  Since  some 
have  spent  3  to  4  cents  a  flat  for  weeding, 
this  claim  is  well  founded.  The  practice 
of  composting,  done  in  part  for  the 
elimination  of  weeds,  is  rendered  un- 
necessary when  soil  is  treated  (Sec.  6). 
Very  few  weeds  survive  heat  treatment  of 
soil  (fig.  70  and  Sec.  9).  and  even  these 
may  be  largely  eliminated  if  germination 
is  started  by  keeping  the  soil  moist  for  a 
few  days  prior  to  mixing  and  treatment. 


SANITARY  PRECAUTIONS  IN  SOIL  TREATMENTS 


To  reduce  the  possibility  of  infesting      treatment.  This  eliminates  handling  and 


treated  soil  with  pathogens,  it  should  be 
h a ndled  as  little  as  possible  after  steam- 
ing or  oilier  trealment.  It  is  desirable, 
therefore,  to  place  the  soil  in  the  con- 
tainers (pots,  fiats,  cans,  benches)  before 


insures  that  the  containers  are  ade- 
quately disinfested.  The  fact  that  soil 
trealment  in  bulk  is  discussed  in  this 
manual  (sees.  10  and  17)  does  not  imply 
that  it  is  equally  satisfactory.  The  method 


120  | 


is  given  because  some  nurserymen  do 
not  find  it  feasible  to  adapt  in-container 
treatment  to  their  operations.  When  bulk 
treatment  is  used,  the  containers  must 
be  separately  treated  and  special  care 
taken  to  avoid  contamination  before  and 
during  filling  operations. 

If  new  cans,  flats,  and  pots  are  used, 
and  these  have  been  carefully  handled  to 
prevent  contamination  after  delivery, 
there  is  less  reason  to  treat  after  filling. 
This  is  also  true  for  new  insert  unit  con- 
tainers for  flats.  If  the  flats  in  which  they 


are  placed  have  been  used  before,  how- 
ever, there  is  still  some  risk.  Further- 
more, the  possibility  of  contamination 
during  the  actual  filling  operation  exists, 
unnecessarily,  in  each  of  these  practices. 
It  is  a  wise  general  practice  to  place 
the  soil  in  containers  and  then  to  treat 
them  as  a  unit. 

Mixing  of  fertilizers  or  other  in- 
gredients into  the  soil  after  treatment 
should  be  avoided;  this  may  readily  be 
accomplished  with  the  U.  C.-type  soil 
mixes  (sees.  5,  6,  and  7). 


TREATMENT  TEMPERATURE  AND  TIME 


A  temperature  of  180°  F  for  30  min- 
utes is  adequate  to  free  soil  of  pathogens, 
weeds,  and  insects  (fig.  70,  and  Sec.  9). 
For  reasons  explained  in  Section  9,  heat- 
ing cannot  be  stopped  at  180°  in  some 
kinds  of  equipment.  A  final  temperature 
of  180°  for  30  minutes  is  possible  and 
satisfactory  for  treatment  of  a  moving 
soil  mass  with  either  a  dry  source  of  heat 
or  steam,  because  the  rate  of  movement 
and  the  amount  of  steam  or  dry  heat  ap- 
plied can  be  regulated.  If  the  English 
steam-air  mixture  system  (Sec.  9) 
should  prove  generally  satisfactory,  this 
temperature  could  also  be  used  for  steam- 
ing a  stationary  soil  mass.  With  present 
methods,  however,  steam  will  not  heat  a 
stationary  soil  mass  to  less  than  212°. 
Hence,  a  final  temperature  of  212°  for 
30  minutes  is  recommended  for  steam 
treatment  of  soil  in  containers  or  in  a 
stationary  bulk  mass. 

These  temperatures  take  into  account 
such  practical  considerations  as  the  re- 
duced rate  of  heat  penetration  into  clods 
and    pockets    of    organic    material    and 

Fig.  70.  Temperatures  necessary  to  kill  path- 
ogens and  other  organisms  harmful  to  plants. 
Most  of  the  temperatures  indicated  here  are 
for  30-min.  exposures  under  moist  conditions. 


°F  O 
212 


200 


190 


180 


170 


160 


150 


140 


130 


120 


110 


100 


M 


Few  resistant  weed  seeds 
Resistant  plant  viruses 


Most  weed  seeds 

_  All  plant  pathogenic  bacteria 
Most  plant  viruses 

.Soil  insects 

Most  plant  pathogenic  fungi 

—  Most  plant  pathogenic  bacteria 
Worms,  slugs,  centipedes 

~  Gladiolus  yellows  Fusarium 

—  Botrytis  gray  mold 

—  Rhizoctonia  solani 

_   Sclerotium  rolfsii  and  Sclerotica sclerotiorum 
~    Nematodes 

—  Water  molds 


[127] 


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plant  residue.  The  timing  should  begin 
when  the  coldest  point  in  the  soil  mass 
has  reached  180°  to  212°  F.  Usually  a 
slight  steam  flow  must  be  maintained 
after  this  time  in  order  to  hold  the  de- 
sired temperature  for  30  minutes.  Equip- 
ment for  treatment  of  bulk  soil  often  will 
have  cold  corners  at  the  bottom.  These 
should  be  located  by  trial,  and  used  as 
the  temperature  test  points.  The  worst 
cold  corners  should  be  eliminated  by 
fitting  the  box  with  triangular  pieces  of 
wood  (see  Sec.  10,  type  4). 

Because  steam  thus  moves  through  the 
soil  as  an  advancing  front,  temperature 
rise  at  a  given  point  is  usually  quite 
sudden  under  efficient  operating  condi- 
tions (fig.  71  and  Sec.  9).  Steam  con- 
denses on  the  nearest  cold  soil  particles 
and  does  not  penetrate  farther  until  it 
has  heated  them  to  212°  F.  For  this  rea- 
son, several  thermometers  may  be  placed 


Fig.  71.  Diagram  showing  movement  of 
steam  from  the  orifice  (or)  in  the  buried  pipe 
(p)  through  a  stationary  soil  mass.  Steam  ex- 
pands as  a  spheroid  with  an  elongated  top. 
If  the  distance  of  movement  (d)  above  the  out- 
let is  1,  then  that  below  it  is  approximately  Vi, 
and  that  to  the  sides  is  Vi  to  %.  An  advancing 
boundary  or  zone  of  condensation  (con)  sepa- 
rates the  unheated  soil  (us)  from  that  at  212°F. 
The  same  unit  at  the  same  instant  of  heating  is 
shown  in  end  view  in  A  and  in  bottom  view  in 
B.  The  shape  of  the  spheroid  would  be  the  same 
in  side  view  as  shown  in  A.  When  the  steam 
from  the  top  pipe  reaches  the  soil  surface  (ss), 
it  escapes  (es).  Steaming  should  then  continue 
at  a  reduced  rate  of  flow  (the  "trickle  finish") 
until  the  cold  corners  (cc)  are  heated.  The  same 
unit  as  in  A  and  B  is  shown  in  C  a  few  minutes 
later,  with  nearly  all  the  soil  heated  to  212°. 
The  zones  do  not  overlap  as  the  steam  flow 
continues,  but  the  steam  then  tends  to  flow  into 
the  cold  corners.  Note  that  the  cold  corners 
are  easily  eliminated  by  a  triangular  piece  of 
wood  (tr)  placed  in  the  vertical  angle  of  the 
box. 


28  I 


at  varying  distances  from  the  steam  in- 
puts to  plot  the  advancing  heat  front.  Ac- 
curate chemical  thermometers  must  be 
used,  since  inexpensive  ones  are  likely 
to  give  inaccurate  readings  (see  Appen- 
dix). Convenient  pellets  called  Tempil 
Pellets  are  available  (Appendix)  that 
melt  at  indicated  temperatures  from 
113°  to  250°  F  or  more.  A  series  that 
melts  at  163°,  188°,  200°,  and  213°  may 
be  placed  in  various  parts  of  the  soil 
mass;  the  highest  one  melted  indicates 
the  highest  temperature  attained  at  that 
point. 

It  is  safe  to  heat  a  U.  C.-type  soil  mix 
to  212°  F  without  the  soil's  becoming 
toxic  to  plants.  With  these  soils  it  is 
better,  therefore,  to  overcook  than  to 
risk  incomplete  heating.  When  possible, 
other  types  of  soils  should  be  heated  to 
only  180°  until  the  grower  finds  that  the 


particular  soil  may  be  safely  heated  to 
212°. 

Temperature  requirement  for 
pinto-tag  certificates 

In  August,  1954,  the  California  De- 
partment of  Agriculture  ruled  that  vege- 
table plants  grown  in  flats  could  be 
moved,  without  inspection  at  destination, 
between  counties  participating  in  the 
intercounty  nursery  stock  certificate 
agreement.  To  qualify,  the  soil  and  flats 
must  be  either  "steamed  in  a  closed 
chamber  until  temperature  of  all  soil 
reaches  180°  F"  or  treated  for  24  hours 
with  methyl  bromide  at  4  pounds  per 
100  cubic  feet,  and  protected  from  recon- 
tamination.  The  certificate  may  be  used 
for  shipments  by  authorized  nurseries  of 
plants  so  handled.  These  "pinto-tag"  cer- 
tificates are  becoming  recognized  as  in- 
dicative of  superior  nursery  stock. 


THE  FORM  OF  STEAM  USED 


Free-flowing   or 
pressureless  steam 

Of  the  three  forms  of  steam  used  for 
soil  treatment,  the  free-flowing  is  the 
most  commonly  used  and  probably,  con- 
sidering equipment  cost,  the  most  prac- 
tical. The  steam  is  at  atmospheric  pres- 
sure or  very  slightly  above.  This  means 
that  large-diameter,  light-weight,  quick- 
coupling  aluminum  irrigation  pipe  can 
be  used  for  mains,  and  that  a  lighter  and 
less  expensive  boiler  is  possible.  There 
are  other  advantages  for  this  type  of 
steam  (sees.  9  and  10).  If  a  boiler  must 
be  purchased  for  soil  steaming,  one  op- 
erating at  little  or  no  pressure  probably 
is  the  best  investment. 

Many  boilers  used  for  glasshouse 
heating  deliver  steam  through  the  mains 
under  pressure.  When  such  steam  is  used 
for  soil  treatment,  it  drops  back  to  at- 
mospheric pressure  as  it  enters  the  soil. 


The  steam  supplied  is  slightly  drier  than 
the  free-flowing  type  and  carries  a  little 
more  heat.  Its  effect  on  the  organisms  is 
the  same  as  the  free-flowing  type,  since 
they  are  subject  to  no  pressure. 

Steam  under  pressure 

Soil  may  be  subjected  to  steam  at  about 
15  pounds'  pressure  in  tight  containers 
(autoclaves  or  cannery  retorts).  Al- 
though there  is  about  1.4  per  cent  more 
B.t.u.  per  pound  in  this  than  in  free- 
flowing  steam,  there  is  no  real  gain  in 
treatment  efficiency  (Sec.  9).  Steam  at 
15  pounds'  pressure  has  a  temperature 
of  249.8°  F.  Since  no  worthwhile  pur- 
pose is  served,  and  the  excessive  heat 
may  be  detrimental  to  the  soil,  the  use 
of  pressure  steamers  is  not  recommended. 
Furthermore,  the  increased  cost  of  equip- 
ment over  the  free-flowing  tvpe  is  ap- 
preciable. 


[129] 


Superheated  steam 

Steam  which  is  heated  to  300°  F  de- 
livers only  4.4  per  cent  more  B.t.u.  per 
pound  than  does  pressureless  steam  (Sec. 
9).  It  has  the  advantage  of  being  drier, 
but    this    is    insufficient    to    justify    the 


greater  boiler  cost.  If  the  soil  is  at  the 
right  moisture  content  before  treating, 
even  free-flowing  steam  does  not  leave 
the  soil  too  wet  for  planting.  Superheat 
boilers  are,  furthermore,  not  used  for 
heating  glasshouses. 


EFFICIENT  SOIL  STEAMING 


To  use  steam  efficiently,  the  rate  of 
flow  should  be  adjusted  to  the  volume  of 
soil  treated,  or  the  volume  of  soil  should 
be  altered  to  the  steam  flow  available. 
The  efficiency  of  the  operation  depends 
on  the  proper  balancing  of  all  the  factors 
by  the  grower. 

Balanced  steaming  and  the 
trickle  finish 

There  are  both  upper  and  lower  limits 
to  the  flow  rate  for  efficient  use  of  steam 
(fig.  74  and  Sec.  9).  If  the  rate  is  too 
high  for  the  volume  of  soil  surrounding 
each  steam  input,  the  steam  will  escape 
or  "blow  out"  from  the  surface  (if  the 
input  is  buried)  or  escape  through  open- 
ings (if  released  into  a  chamber  contain- 
ing soil)  before  some  of  the  soil  reaches 
212°  F.  Steam  will  then  be  wasted.  If  the 
flow  rate  is  too  low,  the  treatment  period 
will  be  unduly  prolonged,  there  will 
probably  be  excessive  loss  of  heat  from 
the  soil  surfaces  before  the  mass  is 
heated,  and  the  soil  may  become  too  wet. 

"Balanced  steaming"  is  achieved  by 
adjusting  soil  volume  and  steam  flow  to 
the  range  of  maximum  efficiency  (Sec. 
9 ) .  This  range  is  affected  by  so  many 
factors  that  no  definite  recommendation 
can  be  made.  It  can  be  readily  found  by 
trial  for  each  set  of  conditions,  since 
there  is  a  fairly  wide  acceptable  range  of 
flow  rates  and  input  spacing  distances. 
The  rate  should  be  such  that  the  tempera- 
ture rise  at  any  point  is  rapid,  once  it  has 
started.  The  best  flow  rate  approximates, 
but   remains  just  below,  that  which  per- 


mits steam  to  escape.  When  only  a  small 
quantity  of  soil  remains  at  a  temperature 
below  212°  F  there  is  an  increasing 
tendency  for  steam  to  escape.  It  is,  there- 
fore, desirable  to  reduce  the  steam  flow 
rate  at  this  time,  the  so-called  "trickle 
finish."  After  the  coldest  corners  have 
reached  212°,  the  steam  need  be  kept  on 
only  enough  to  prevent  the  temperature 
from  dropping  below  180°  during  the 
next  30  minutes. 

Preventing  "blow-out" 

Several  additional  precautions  are 
helpful  in  preventing  "blow-out"  and  ex- 
cessive steam  loss. 

The  soil  surface  should  be  level,  and 
of  uniform  height  above  the  steam  out- 
lets, so  that  steam  will  not  reach  the  sur- 
face at  one  point  and  escape,  which 
would  decrease  penetration  elsewhere. 

The  soil  should  be  well  mixed  and  of 
uniform  moisture  and  compaction.  Those 
areas  of  beds  which  have  been  walked 
on,  or  which  are  wetter  than  surrounding 
soil,  will  heat  more  slowly  than  the  rest. 
Steam  will,  therefore,  escape  at  the  other 
points  before  these  are  heated. 

The  introduction  into  soil  of  steam 
under  fairly  high  pressure  may  cause  it 
to  form  a  channel  to  the  surface.  In  such 
cases,  placing  a  pressure  reducer  in  the 
line  will  be  helpful. 

Depth   and   spacing   of 
steam   outlets 

The  depth  and  spacing  of  steam  out- 
lets in  a  stationary  mass  of  soil  is  im- 


I  L30  I 


portant  for  the  most  efficient  treatment. 
They  are  determined  in  part  by  available 
steam  flow.  The  lower  the  flow  rate,  the 
shallower  should  be  the  soil  layer,  and 
the  closer  the  spacing  of  pipes  and  the 
holes  in  them. 

In  a  unit  with  a  single-layer  steam 
grid  (Sec.  10,  type  1)  the  pipes  should 
be  so  placed  that  the  distance  upward  to 
the  soil  surface  is  at  least  twice  that  to 
the  bottom  of  the  container.  In  many 
such  units  it  is  placed  on  the  bottom.  This 
is  in  accordance  with  the  tendency  of 
heat  to  rise.  If  the  pipe  is  placed  too  close 
to  the  surface,  the  steam  will  escape  be- 
fore most  of  the  soil  is  heated. 

In  a  unit  with  a  multilayer  steam  grid 
(Sec.  10,  type  4)  the  distance  between 
layers  should  be  1%  times  that  of  the 
depth  of  the  top  layer,  and  the  distance 
from  the  lowest  pipes  to  the  bottom  of  the 
soil  should  be  %  the  depth  of  the  top 
layer.  This  is  because  steam  moves  into 
soil  from  the  outlet  in  the  shape  of  a 
spheroid,  enlongated  at  the  top,  as  shown 
in  figure  71  and  explained  in  Section  9. 
When  the  zones  of  condensation  in  the 
expanding  spheres  meet  they  do  not  over- 
lap, and  the  steam  probably  then  flows 
toward  the  unheated  corners.  When  all 
soil  is  heated  to  212°  F,  the  steam  will 
diffuse  out  of  the  soil  mass  at  every  pos- 
sible point. 

The    distance    between    steam    outlets 


able.  The  examples  given  in  Section  9 
will  provide  a  basis  for  estimating  proper 
spacing  of  outlets  for  a  given  boiler 
capacity. 

Steam  applied  to  soil  surface 

When  steam  is  applied  to  the  sur- 
face of  soil  by  either  the  Thomas  or  in- 
verted-pan method  (Sec.  10,  types  18  and 
19)  the  dependable  depth  of  steam  pene- 
tration is  about  8  inches.  If  this  depth  is 
exceeded,  steam  escapes  and  does  not 
move  efficiently  through  soil.  This  depth 
is  adequate  for  most  benches.  It  may  be 
insufficient  for  ground  beds  to  be  used 
for  deep-rooted  plants;  in  such  cases  it 
is  better  to  inject  steam  into  the  soil 
through  buried  perforated  pipes,  buried 
tiles,  or  moving  rakes  (Sec.  10,  types  20, 
22,  and  23). 

Steam   applied   to   soil 
in   chambers 

When  pressureless  steam  is  released 
into  a  chamber  surrounding  soil  in  con- 
tainers (Sec.  10,  types  4b,  5,  6,  7,  8,  10, 
11,  12,  and  13),  it  is  important  that  the 
containers  be  separated  by  about  % 
inch  in  each  direction.  A  method  for 
stacking  flats  is  shown  in  figure  104 
that  permits  unrestricted  steam  flow 
without   use   of   separator   strips.   Many 


and  between  pipes  should  not  be  more      nurseries  prefer  to  place  wooden  sepa- 


than  25  per  cent  greater  than  the  depth 
of  the  pipe  (or  of  the  top  pipe  in  a 
multilayer  grid).  The  pipes  may  be 
closer  together  than  this,  but  they  should 
not  be  so  close  that  the  soil  bridges  on 
them  when  it  is  dumped. 

Decreasing  the  steam  flow  rate  and  in- 
creasing the  distance  between  steam  out- 
lets have  the  same  retarding  effect  on 
temperature  rise  in  the  soil  mass  (fig.  76 
and  Sec.  9).  If  a  boiler  of  small  output 
volume  is  used,  therefore,  efficiency  will 
demand  a  closer  spacing  of  pipes  and 
orifices  in  the  soil  mass  than  will  be 
necessary  if  a  high-output  boiler  is  avail- 


rator  strips  between  horizontal  layers  of 
flats,  and  to  leave  a  small  crack  between 
vertical  piles.  All  chamber  steamers  have 
little  escaping  steam  as  long  as  there  is 
cold  soil  for  it  to  condense  on.  As  ex- 
posed surfaces  are  heated,  however,  and 
steam  must  penetrate  farther  through 
soil  before  condensing,  it  escapes  in- 
creasingly from  the  chamber.  For  this 
reason,  it  is  inefficient  to  steam  soil  in 
very  large  containers  (such  as  tubs  or 
large  planter  boxes)  by  this  method. 
There  is  satisfactory  efficiency  in  flats  up 
to  4  inches  deep,  since  the  steam  then 
only   penetrates   2   inches   through   soil. 

[131] 


Efficiency  will  be  improved  by  reducing 
steam  flow  when  excessive  steam  loss 
from  the  chamber  indicates  that  the  con- 
densation rate  of  the  soil  has  been  ex- 
ceeded. 

When  steam  is  maintained  under  pres- 
sure  in    a   chamber    (Sec.    10,   type   9) 


there  is  no  problem  of  its  escaping  when 
the  condensing  power  of  the  soil  is  ex- 
ceeded. Before  beginning  soil  treatment, 
the  air  must  be  fully  displaced  from  the 
autoclave  by  steam,  in  order  to  prevent 
cold  pockets.  If  this  is  carefully  done  it 
will  not  entail  excessive  steam  loss. 


VOLUME  OF  STEAM  REQUIRED 


The  quantity  of  steam  produced  by 
the  boiler  and  delivered  to  the  soil 
largely  determines  the  quantity  of  soil 
that  can  be  heated,  and  the  time  required 
to  do  it.  It  is,  therefore,  desirable  to  have 
a  boiler  and  steam  mains  of  such  ca- 
pacity that  the  flow  rate  of  steam  does 
not  seriously  limit  the  operation.  The  in- 
formation supplied  in  table  14  and  Sec- 
tion 9  makes  it  possible  to  estimate  the 
size  of  boiler  required  for  a  given  opera- 
tion. The  services  of  a  heating  engineer 
will  be  helpful  in  calculating  the  capacity 
of  the  boiler  required  for  a  specific  situa- 
tion, taking  into  account  the  efficiency  of 
equipment  used. 

If  the  operation  is  fairly  simple,  or  if 
the  adequacy  of  an  existing  boiler  is  in 
question,  the  capacity  needed  can  be 
roughly  determined  from  the  volume  of 
soil  to  be  heated  at  one  time.  The  follow- 
ing examples  give  approximate  calcula- 
tions of  steam  requirements  for  various 
types  of  operations.  More  exact  methods 
of  calculation  are  presented  later  in  this 
section. 

From  table  14  in  Section  9  it  can  be 
calculated  that  approximately  6.5  pounds 
of  steam  will  be  required  per  cubic  foot 
of  a  U.  C.-type  soil  mix  to  raise  the 
temperature  150  degrees  F  in  equipment 
of  about  50  per  cent  efficiency.  Such  ef- 
ficienc)  is  attained  or  exceeded  by  equip- 
ment in  which  steam  is  injected  into  the 
stationary  soil  mass  (sees.  9  and  10). 
Examples  are  the  Rudd  type  (type  1  ). 
steam  boxes  (4a  and  4b),  buried  per- 
forated pipe  (20),  and  buried  tile  (22). 


The  mobile  bin  (2)  and  combined  bin 
and  potting  bench  (3)  may  attain  such 
efficiency  if  heat  loss  from  the  sides  and 
bottom  is  not  excessive. 

Thus,  if  a  steam  box  containing  25 
cubic  feet  of  soil  is  to  be  heated  from  62° 
to  212°  F,  about  163  pounds  of  steam 

(25  x  6.5  x  )  will  be  required.  A 

150/ 

boiler  delivering  200  pounds  per  hour 
will  be  ample  if  long  pipe  and  hose  con- 
nections are  avoided,  and  if  almost  an 
hour  can  be  devoted  to  the  heating.  If 
steaming  needs  to  be  done  in  a  half 
hour,  a  boiler  delivering  400  pounds 
might  be  required. 

Similarly  it  can  be  calculated  that  ap- 
proximately 10.8  pounds  of  steam  will 
be  required  per  cubic  foot  of  mix  in 
equipment  of  30  per  cent  efficiency.  Such 
efficiency  levels  may  be  reached  or  ex- 
ceeded with  chamber-type  equipment 
when  steam  is  released  into  space  sur- 
rounding containers  of  soil  (sees.  9  and 
10).  Examples  are  the  Thomas  method 
(types  5  and  18),  vault  (6),  multipur- 
pose tank  (7),  vertical  cabinet  (8),  and 
inverted  pan  (19). 

Thus,  if  a  bench  of  soil  3  x  10  feet  x  8 
inches  deep  is  to  be  steamed  by  the 
Thomas  method,  and  the  soil  tempera- 
ture must  be  raised  170  degrees  F  from 
42°,  about  245  pounds  of  steam  will  be 

170\ 


required  (  3  x  10  x  %  x  10.8  x 


1 50 


boiler   delivering   300   pounds   of  steam 
would  be  adequate  to  treat  the  soil  in  an 


[132] 


hour,  and  600  pounds  would  be  needed 
for  a  half-hour  treatment. 

Similarly,  a  pile  of  100  flats,  each 
18  x  18  x  3  inches,  heated  140  degrees  F 
from  72°,  would  require  about  567 
pounds  of  steam  calculated  as  follows: 

fiy2  x  1%  x  14  x  100  x  10.8  x  l^\  .  A 

boiler  delivering  600  pounds  would 
heat  these  in  an  hour,  and  a  1,200-pound 
unit  in  30  minutes. 

Among  factors  that  the  grower  must 
consider  in  selecting  a  boiler  and  treat- 


ing equipment  is  the  amount  of  soil  that 
must  be  steamed  per  day.  Soil  can  be 
treated  in  successive  batches,  but  one 
should  be  certain  that  enough  batches 
can  be  treated  in  a  working  day.  If  it 
takes  an  hour  to  bring  the  soil  to  212°  F, 
plus  the  required  half-hour  exposure, 
and  an  hour  to  cool,  unload,  and  reload 
the  unit,  only  3  batches  can  be  treated  in 
an  8-hour  day.  In  designing  the  steam- 
ing operation,  the  amount  of  soil  per 
load  and  per  day  is  the  proper  starting 
point  in  calculations. 


PREPARING  SOIL  FOR  STEAMING 


Moisture  content 

The  moisture  content  of  soil  to  be 
heated  is  very  important  in  determining 
the  efficiency  of  the  operation.  It  requires 
about  five  times  as  much  heat  to  raise 
the  temperature  of  a  pound  of  water  1 
degree  F  as  it  does  a  pound  of  soil  (Sec. 
9) .  On  the  other  hand,  heat  plus  moisture 
is  much  more  effective  in  killing  patho- 
gens than  is  heat  alone.  Seeds  start  to 
germinate  if  kept  wet  a  few  days  prior  to 
treatment,  and  even  the  more  resistant 
weeds  (Sec.  9)  are  then  killed  by  treat- 
ment. 

These  objectives  are  served  if  the  soil 
to  be  treated  is  moist  enough  for  plant- 
ing. After  it  is  squeezed  in  the  hand  it 
should  crumble  freely. 

Dripping  benches  during  steaming  in- 
dicate that  the  soil  was  initially  too  wet, 
that  the  steam  carried  condensed  water, 
or  perhaps  that  the  flow  rate  was  insuf- 
ficient. 

If  the  soil  is  very  wet  when  steamed  it 
may  come  out  in  a  soggy  condition,  be- 
cause the  water  held  increases  the  B.t.u. 
requirement  and  more  steam  must  be 
used,  which  adds  more  water  from  con- 
densation, in  a  vicious  spiral.  This  will 


be  aggravated  if  the  excess  water  does 
not  drain  off  readily,  as  in  some  ground 
beds  or  heavy  soil  where  evaporation  is 
not  rapid. 

It  is  always  advisable  to  bleed  the 
water  and  moist  steam  from  the  pipe  line 
and  hose  near  the  point  of  connection  to 
the  soil  before  treatment  begins.  If  this  is 
not  done,  water  is  initiallv  discharged 
into  the  soil,  causing  increased  B.t.u.  re- 
quirement, uneven  soil  heating,  and  wet 
soil.  For  the  same  reason  it  is  desirable 
to  use  a  water  trap  in  the  steam  line  near 
the  point  of  entry  into  the  soil,  so  as  to 
continuously  drain  off  the  condensate. 
This  is  particularly  important  if  long 
mains  are  used. 

Freedom  from  lumps 

U.  C.-type  mixes  use  fine  sand  of  a 
type  which  does  not  form  hard  clods 
(Sec.  6).  Even  these,  however,  need  to 
be  well  mixed  to  avoid  pockets  of  peat. 
If  lumpy  soil  is  used,  it  should  be  pulver- 
ized or  screened  before  being  steamed. 
This  is  because  clods  are  not  readily 
penetrated  by  steam  I  Sec.  9).  Uneven 
packing  of  the  soil  in  the  container  also 
makes  for  uneven  heating. 


[133] 


UNEVEN  HEATING 


If  soil  heats  unevenly  during  treat- 
ment, one  of  the  following  factors  may 
be  the  cause. 

Uneven  compaction 

Areas  of  beds  which  have  been  walked 
or  driven  over  will  heat  more  slowly  than 
the  rest,  owing  to  reduced  porosity  of  the 
soil.  The  trouble  is  aggravated  at  high 
rates  of  steam  flow.  The  presence  of 
clods,  just  mentioned,  also  affects  heat 
penetration.  Soil  in  benches  or  beds 
should  be  well  prepared  and  free  of  clods 
to  eliminate  areas  of  compaction.  In 
boxes  and  bins  one  should  not  tamp  the 
soil,  but  let  it  settle  firmly  into  place. 

Uneven  moisture 

Soil  with  uneven  moisture  distribution 
will  heat  unevenly,  because  of  the  greater 
specific  heat  of  water  than  soil.  Soil 
should  never  be  watered  after  prepara- 
tion and  before  steaming.  Proper  prepa- 
ration will  do  much  to  make  moisture 
satisfactorily  uniform. 

Cold  corners 

"Cold  corners"  in  boxes  or  bins  have 
been  discussed  above  and  diagrammed 
in  figure  71.  The  problem  may  be  elimi- 
nated by  filling  each  corner  with  a  tri- 
angular wood  block. 


Excessive  spacing  of  pipes 

If  pipes  in  a  grid  are  too  far  apart, 
relative  to  the  steam  flow  rate,  heating 
will  be  so  retarded  that  unevenness  may 
result,  particularly  if  a  clod  or  compact 
or  wet  area  occurs  at  the  cool  spot. 

Uneven  mixing 

If  the  peat  or  other  organic  material 
is  not  evenly  mixed  with  the  fine  sand, 
uneven  heating  may  occur.  Pockets  of 
dry  peat  are  relatively  impenetrable  by 
heat,  owing  to  their  insulating  properties. 

Insufficient  steam 

If  the  flow  rate  is  insufficient  for  the 
volume  of  soil  treated,  trouble  with  un- 
even heat  is  aggravated.  See  "Efficient 
Soil  Steaming,"  above. 

Partially  frozen  soil 

Attempting  to  steam  partially  frozen 
soil  leads  to  uneven  heating.  It  should 
be  thawed  throughout,  and  mixed  before 
steaming. 

Unexpelled  air  in  autoclaves 

Uneven  heating  in  autoclaves  results 
when  the  air  is  not  expelled  before  in- 
creasing the  pressure.  The  air  should  be 
forced  out  of  the  exhaust  valve  or  par- 
tially open  door  by  incoming  steam  for 
several  minutes,  before  closing  the  auto- 
clave and  beginning  the  treatment. 


COOLING  THE  TREATED  SOIL 


Our  experience  is  that  flats  may  drop 
to  temperatures  suitable  for  planting 
within  1  or  2  hours,  the  time  depending 
upon  exposure,  air  movement,  and  air 
temperature. 

The  process  of  evaporation  accelerates 
the  cooling  of  the  soil.  In  uncovered  beds 
the  temperature  has  been  reported  to 
drop  from  212'   F  to   160°  in  1  minutes 


at  the  surface,  2.6  hours  at  the  2-inch 
level,  and  8.3  hours  at  the  7-inch  level.  A 
layer  of  canvas  may  extend  the  time  of 
cooling  at  the  surface  to  1 1  minutes. 

Whenever  treated  soil  is  dumped  and 
piled  on  the  floor,  the  surface  should 
previously  have  been  wet  down  with  a 
formaldehyde  solution  (1  gal.  to  18  gal. 
water). 


I  K?41 


WATER  CONTENT  AFTER  STEAMING 


Growers  often  ask  whether  steaming 
does  not  make  the  soil  excessively  wet  to 
use.  It  may  be  calculated  that,  to  raise  the 
temperature  of  1  cubic  foot  of  soil  with 
15  per  cent  moisture  150  degrees  F, 
would  add  4.1  pounds  of  water,  or  6.8 
per  cent,  through  condensation  of  steam 
(assuming  80  per  cent  efficiency  in  heat 
transfer).  Senner  (1934)  showed  that  the 
moisture  in  soil  after  steaming  in- 
creased by  2.3  to  8  per  cent.  Bunt  (1954- 
55)  found  an  increase  of  2.0  to  5.6  per 
cent,  and  Morris  (1954a)  reported  5  to 
7  per  cent  for  light  and  7  to  12  per  cent 
increase  for  heavy  soil. 

Actually  much  of  this  added  moisture 
is  lost  by  evaporation  as  the  soil  cools. 
Thus,  in  our  tests  with  U.  C.  mix  B  (25 


per  cent  peat)  the  final  soil  moisture  was 
increased  by  only  2.1,  3.6,  and  3.9  per 
cent  after  cooling.  Even  if  evaporation  is 
reduced  by  stacking  the  flats  or  covering 
the  soil  mass,  the  moisture  content  of  a 
U.  C.-type  mix  has  never  been  excessive 
after  treatment.  For  this  reason  we  rec- 
ommend that  the  soil  to  be  steamed 
should  be  moist  enough  to  plant  prior  to 
treatment. 

There  is  no  consistent  difference  in  the 
final  moisture  content  of  soil  treated  by 
free-flowing  and  pressure  steam,  and  this 
is  unchanged  by  rate  of  steam  flow  ex- 
cept at  very  low,  inefficient  levels,  where 
it  increases.  Soil  treated  with  super- 
heated steam  will  be  slightly  drier  (Sec. 
9). 


STEAM-TREATING  HOME  YARDS 


The  fact  that  soil  may  be  treated  in 
proximity  to  plants  suggests  the  pos- 
sibility of  the  use  of  steam  in  home  yards 
against  infestations  of  the  oak-root 
fungus,  aster-wilt  Fusarium,  water  molds, 
and  similar  persistent  soil  fungi.  A  porta- 
ble steam  boiler  and  a  steam  rake  with 
pan  (Sec.  10,  type  21 1,  could  be  used  to 
insure  depth  of  penetration. 


Such  a  practice  would  enable  the 
progressive  nurseryman  to  cope  with  the 
problem  of  existing  infestation  of  soil  in 
home  yards,  so  that  the  healthy  plants 
he  sells  will  not  be  killed  as  previous 
ones  have  been  when  planted  there.  This 
investment  in  good  will  might  also  prove 
to  be  profitable. 


COST  OF  STEAMING  SOIL 

Because  of  the  variation  between  nurseries  in  many  operating  conditions  that 
greatly  influence  the  cost  of  soil  steaming,  no  exact  figure  can  be  given  for  all  situa- 
tions. A  hypothetical  conservative  example  is  given  in  detail  so  that  the  grower  may 
compare  his  own  operations  with  it,  and  calculate  his  approximate  cost. 

Heat  requirements 

Assuming  a  soil  with  a  specific  heat  of  0.2,  15  per  cent  moisture  content,  a  weight 
of  60  lb.  per  cu.  ft.,  and  the  temperature  to  be  raised  150  degrees  F  (for  example 
from  62°  to  212°  F),  and  water  with  a  specific  heat  of  1.0: 

B.t.u.  for  1  cu.  ft.  soil  =  60  lb  x  0.2  specific  heat  x  150°  F =  1.800 

B.t.u.  for  water  =  60  lb.  x  0.15  moisture  x  1.0  specific  heat  x  150°  F  =  1.350 


B.t.u.  requirement  per  cu.  ft. 


3.150 


[135] 


Assuming  gas  fuel  of  1,100  B.t.u.  per  cu.  ft.  heat  value  and  different  levels  of 
efficiency  in  the  total  heat  exchange:  At  100  per  cent  efficiency,  the  gas  used  per  cu. 

3150 

ft.  of  soil  is  _  ■,-■.. — z— -  =  2.86  cu.  ft.;  at  70  per  cent  it  is  4.09  cu.  ft.;  at  50  per 
1100  x  1.00 

cent  it  is  5.73  cu.  ft.;  at  30  per  cent  it  is  9.55  cu.  ft. 

Cost  of  fuel 

Using  the  most  expensive  (winter)  gas  rates  (G-40  schedule),  and  assuming  a 
nursery  of  60,000  flats  per  year,  operating  at  3  levels  of  efficiency  in  heat  exchange: 

60,000  flats  ==  1,000  cu.  yd.  =  27,000  cu.  ft.  of  soil. 
At  70  per  cent  efficiency: 

27,000  cu.  ft.  soil  x  4.09  —  110,430  cu.  ft.  gas  required. 

100,000  cu.  ft.  at  $0,564  per  thousand  rate =  $56.40 

10,430  cu.  ft.  at  $0,514  per  thousand  rate =      5.36 

Cost  for  27,000  cu.  ft.  soil =  $61.76 

Cost  per  cu.  ft.  soil =$  0.002237 

Similarly,  at  50  per  cent  efficiency: 

Cost  per  cu.  ft.  soil =  $  0.003130 

Similarly,  at  30  per  cent  efficiency : 

Cost  per  cu.  ft.  soil =  $  0.005094 

Cost  per  cu.  yd.  soil =  $  0.13754 

Thus,  with  gas,  the  cost  of  fuel  for  steaming  soil  ranges  from  about  0.23  cent  per 

cubic  foot  at  70  per  cent  efficiency  up  to  0.51  cent  at  30  per  cent  efficiency.  Fuel  oil, 

costing  about  0.078  cent  per  1,000  B.t.u.,  as  against  about  0.055  cent  for  natural 

gas,  would  make  the  cost  about  half  again  as  much  for  fuel. 

Cost  of  equipment 

If  a  boiler  is  already  used  for  heating  glasshouses,  this  cost  may  be  prorated  and 
will  be  less  than  the  figures  quoted  below.  These  calculations  are  based  on  the  pur- 
chase of  a  small  boiler,  distribution  lines,  and  soil-steaming  equipment  specifically 
for  this  operation.  A  25-day  working  month  is  assumed,  with  3  batches  of  soil  per 
day. 

27,000  cu.  ft.  per  yr.  =  2,250  cu.  ft.  per  month 

2,250  cu.  ft. 

— ^r— j =  90  cu.  ft.  per  day 

2d  days 

90  cu.  ft. 

TTi j —  =  30  cu.  ft.  per  batch,  which  must  be  heated  in  1  hr. 

o  batches 

This  would  require  324  lb.  of  steam  per  hr.  at  30  per  cent  efficiency  (30  cu.  ft.  x 

10.8  lb.  per  cu.  ft.;  see  "Volume  of  Steam  Required"). 

Since    efficiency    has    already    been    calculated,    the    boiler    would    need    to   be 

324  lb. 

— — —  -,  or  9.4  horsepower.  A  10  to  15  horsepower  boiler  that  would  produce  around 

34.5  lb. 

400  lb.  steam  per  hr.  would  thus  be  adequate  for  this  job. 


[136] 


Equipment  cost  of  $3,000,  with  a  10-year  life,  and  a  6  per  cent  interest  rate  is 

assumed  to  provide  the  needed  equipment: 

...   S3000 

Principal,  cost  per  year  (depreciation)  - =  $300 

10  yr. 

Interest  at  6  per  cent,  average  per  year =      99 

Equipment  cost  per  year =  $399 

$399 

Cost  of  equipment  per  cu.  ft.  -  =  $     0.014778 

H    ^  F  27,000 

Total  cost 

Any  additional  labor  introduced  by  the  steaming  operation  is  here  omitted  because 
of  the  extreme  variability  in  various  nurseries.  Furthermore,  most  of  the  handling 
in  a  bedding-plant  or  gallon-can  nursery  will  be  necessary  whether  the  soil  is 
steamed  or  not. 

Cost  of  equipment  per  cu.  ft =  $0.014778 

Cost  of  fuel  (at  30  per  cent  efficiency)  per  cu.  ft =  $0.005094 

Cost  per  cu.  ft.  of  soil,  exclusive  of  labor ===  $0.019872 

It  should  be  emphasized  that  these  figures  are  very  conservative,  being  based  on 
only  30  per  cent  heat  efficiency,  on  gas  at  the  winter  rates,  and  include  cost  of  the 
boiler  and  equipment.  The  calculated  cost  of  steaming  is  appreciably  below  that  of 
methyl  bromide  or  chloropicrin  fumigation  (table  13),  even  when  no  equipment 
cost  is  included  for  those  treatments.  Since  the  labor  would  be  approximately  the 
same  for  each  of  the  three  methods,  it  may  be  disregarded.  It  is  clear  that  soil  steam- 
ing is  cheaper  than  fumigation,  even  when  a  boiler  and  equipment  must  be  purchased. 
The  argument  of  the  initial  cost  of  the  boiler,  usually  cited  as  the  reason  for  fumigat- 
ing rather  than  steaming,  is  shown  to  be  economically  unsound,  even  if  money  has 
to  be  borrowed  to  purchase  the  equipment. 


[137] 


SECTION 


Principles  of  Heat 
Treatment  of  Soil 


Kenneth  F.  Baker 


Chester  N.  Roistacher 


Temperature  and  time  necessary  for  treatment 
Objectives  and  definitions 
Treatment  of  soil  by  heat 
Treatment  of  soil  by  hot  water 
Treatment  of  soil  by  steam 


HE  IMPORTANCE  of  the  soil  in  plant 
culture  has  long  been  recognized  by  com- 
mercial growers  and  botanists.  It  actually 
supplies  materials  which  make  up  about 
85  per  cent  of  the  weight  of  green  plants 
( 80  per  cent  water,  2  per  cent  minerals, 
and  3  per  cent  as  hydrogen  and  oxygen 
in  carbohydrates),  with  only  15  per  cent 
taken  from  the  air  (carbon  and  oxygen 
in  carbohydrates).  The  justifiable  early 
practical  interest  in  soils  and  in  "root 
action"  quickly  extended  to  include  root 
parasites  after  1850-1880,  once  it  had 
been  clearly  established  that  plant  dis- 
eases could  be  caused  by  microorgan- 
isms. Attempts  were  soon  made  to  free 
the  soil  of  organisms  injurious  to  plants 
by  treating  it  with  heat  or  chemicals. 

Heat    sterilization    had    been    demon- 
strated   to   destroy    fermentation    organ- 
isms  as   early    as    1776,   and    to    destroy 
fungi    in    living    plant    tissue    by    1883. 
\bout    L890,   aspetic   surgery,    involving 


heat  sterilization  of  equipment,  came  to 
the  fore.  About  the  same  time  (for  exam- 
ple, by  B.  Frank,  1888,  in  Germany) 
steam  treatment  of  soil  was  experi- 
mentally used.  W.  N.  Rudd  (1893)  in 
Mt.  Greenwood,  Illinois,  commercially 
injected  steam  through  perforated  buried 
pipes  in  the  bottom  of  a  bin  of  soil  to  kill 
fungi,  weeds,  and  insects,  and  similar 
methods  were  soon  adopted  by  other 
growers.  The  Wutrick  Brothers,  Cleve- 
land, Ohio,  are  said  to  have  used  the 
steam-pan  method  about  this  same  time. 
It  is  to  be  noted  that  florists  and  nursery- 
men were  only  a  few  years  behind  the 
medical  profession  in  the  adoption  of 
steam  sterilization.  It  has  now  become  a 
standard  procedure  in  glasshouse  opera- 
tions the  world  over  in  order  to  reduce 
losses  from  diseases,  weeds,  and  insects. 
This  section  presents  the  principles 
and  data  supporting  the  practices  out- 
lined   in   Section   8   and    the   equipment 


I  1^8  3 


described  in  Section  10  for  the  heat 
treatment  of  soil.  This  background  in- 
formation will  provide  a  better  basis  for 
present  use  of  the  suggestions  outlined 
in  the  preceding  section,  and  is  neces- 


sary to  the  understanding  of  new  de- 
velopments. For  example,  the  factual 
basis  for  the  use  of  steam-air  mixtures 
being  developed  in  England  is  necessary 
to  understand  and  evaluate  this  method. 


TEMPERATURE  AND  TIME  NECESSARY  FOR  TREATMENT 


Extensive  studies  by  many  workers  in 
various  parts  of  the  world  have  demon- 
strated that  exposure  to  moist  heat  at 
150°  F  for  30  minutes  will  destroy  the 
important  plant  pathogens,  insects,  and 
weeds  (fig.  70) . 

Fungi . . . 

are  relatively  sensitive  to  heat.  Rhizoc- 
tonia  may  be  eradicated  from  living 
plant  tissue  by  hot-water  treatment  at 
125°  F  for  30  minutes  (Sec.  13)  ;  the 
most  rigorous  treatments  recommended 
have  been  122°  for  60  minutes.  Water 
molds  are  even  more  sensitive,  Pythium 
ultimum  being  killed  in  Aloe  and  Haw- 
orthia  plants  at  115°  in  20  to  40  minutes 
(Sec.  13).  The  Botrytis  gray  mold  is 
killed  at  131°  for  15  minutes.  The 
gladiolus-yellows  Fusarium  is  killed  in 
cormels  at  135°  for  30  minutes.  The 
cottony-rot  Sclerotinia  is  destroyed  at 
122°  for  5  minutes.  Sclerotium  rolfsii  is 
killed  in  30  minutes  at  122°  in  caladium 
tubers  and  iris  rhizomes  (Sec.  13).  Most 
other  pathogenic  fungi  are  also  destroyed 
by  time-temperature  relations  below 
140°  for  30  minutes. 

Bacteria 

Most  bacteria  that  cause  plant  disease 
are  killed  at  140°  F  for  10  minutes,  and 
probably  all  at  160°,  since  they  do  not 
form  the  heat-resistant  spores  of  some 
animal  pathogens  and  food-spoilage 
forms.  The  data  in  figure  68  also  show 
that  steaming  at  212°  for  30  minutes  was 
as  effective  as  8  hours  in  destroying  the 
spore-forming  ammonifiers. 


Nematodes . . . 

are  also  quite  susceptible  to  heat.  The 
root-knot  nematode  is  killed  at  118° 
F  in  10  minutes  and  is  easily  eradicated 
in  living  plants  (Sec.  13).  The  most 
resistant  foliar  nematodes  are  killed  at 
120°  for  15  minutes.  The  stem  and  bulb 
nematode  is  killed  at  127°  for  11  min- 
utes. The  resistant,  cyst-forming  potato 
root  nematode,  not  known  in  California, 
is  killed  at  118°  for  15  minutes.  The 
lesion  nematodes  are  killed  at  120°  for 
10  minutes. 

Insects  and  mites  . . . 

are  also  susceptible  to  heat,  even  in 
the  egg  stage,  and  cannot  long  survive 
140°  to  160°  F.  Worms,  slugs,  centi- 
pedes, and  similar  animals  are  ap- 
parently destroyed  by  moist  heat  at  140° 
for  30  minutes. 

Weeds . . . 

for  the  most  part,  are  destroyed  at 
temperatures  of  158°  to  176°  F  for  15 
minutes.  In  California  nursery  expe- 
rience, however,  three  weeds  survive  tem- 
peratures approaching  212°;  these  are 
the  button  weeds  (Malva),  bur  clover 
{Medicago),  and  Lotus  strigosus.  In 
other  areas,  shepherd's  purse  {Cap  sell  a  ) . 
Klamath  weed  (Hypericum),  lambs 
quarter  (Chenopodium) ,  wild  oat 
(Avena),  and  some  mustards  are  re- 
ported to  be  quite  heat-tolerant.  Expe- 
rience has  indicated  that  seed  of  these 
plants  is  not  numerous  in  soils  of  the 
type  used  in  California.  A  fairly  satis- 
factory  index   of  effectiveness   of  treat- 


[139] 


ment  used  by  California  nurserymen  is 
whether  weeds  other  than  the  first  three 
mentioned  appear  after  steaming. 

Viruses . . . 

of  nursery  plants  do  not  persist  in  soil, 
but  some  may  survive  in  undecom- 
posed  plant  refuse  for  a  time.  Thus,  the 
virus  of  chrysanthemum  virus  stunt  will 
live  over  in  dried  infected  tissue  for  at 
least  2  years,  and  will  survive  200°  F 
for  10  minutes.  In  spite  of  this,  soil 
carryover  of  this  virus  is  adequately 
eliminated  in  commercial  operations  by 
removal  of  most  of  the  plant  residue  and 
decay  of  the  rest,  and  by  steaming. 
Tobacco  mosaic,  a  similar  virus  with 
respect  to  carryover,  has  been  intensively 
studied  in  the  Department  of  Plant 
Pathology  glasshouses  at  the  University 
of  California,  Los  Angeles,  during  the 
past  7  years;  the  above  procedures  have 
been  so  successful  that  there  has  been  no 
soil  carryover,  despite  the  fact  that  212° 
moist  heat  for  15  minutes  is  required  to 
destroy  the  virus  in  dead  stems.  The 
majority  of  viruses  are  destroyed  by  tem- 
peratures of  about  160°  for  30  minutes 
and  do  not  survive  in  the  soil  or  refuse. 
The  use  of  a  U.  C.-type  soil  mix  also 
virtually  precludes  any  virus  carryover 
because  no  host  plants  of  troublesome 
viruses  occur  in  the  source-areas  of  the 
ingredients.  Experience  has  shown  that 
growers  following  the  U.  C.  system  of 
soil  mixes  and  soil  treatment  have  no 
trouble  with  virus  carryover. 

Recommendations 

Under  ideal  conditions,  the  organisms 
of  concern  to  growers  may  be  killed  by 
heating  to  140°  F  for  30  minutes,  a  fact 
championed  by  A.  G.  Newhall  at  Cornell 
University  over  the  last  two  decades. 
Since;  commercial  operations  do  not  sup- 
ply ideal  conditions,  a  compromise  with 
reality  is  necessary.  For  example,  clods 
or  lumps  do  not  heat  through  as  quickly 


as  loose  soil,  and  there  may  be  "cold 
corners"  in  the  equipment.  To  give  a 
working  margin  of  safety  we  have,  for 
the  past  16  years,  recommended  a  mini- 
mum temperature  of  180°  for  30  min- 
utes. Before  the  development  of  the 
U.  C.-type  mix,  there  was  some  danger 
of  soil  post-steaming  toxicity  (Sec.  6), 
and  it  was  therefore  desirable  to  keep 
the  temperature-time  as  low  as  possible. 
Now  that  this  is  no  longer  a  factor,  even 
higher  temperatures  are  safe. 

The  present  recommendation  is  to  heat 
a  soil  mix  of  the  U.  C.  type  to  212°  F  for 
30  minutes,  except  that  with  equipment 
in  which  uniform  heating  can  be  stopped 
at  180°  and  held  there  for  30  minutes,  it 
is  safe  to  do  so.  The  temperature  of  212° 
is  specified  for  steam  because:  (1)  it  is 
often  the  only  possible  final  temperature 
for  the  process;  (2)  it  is  a  temperature 
that  can  be  easily  controlled  (soil  tem- 
perature is  not  raised  above  that  point 
by  steam,  except  under  superheating  or 
pressure)  ;  (3)  a  U.  C.-type  mix  develops 
no  post-steaming  toxicity  for  plants  and 
may  safely  be  heated  to  212°;  (4)  the 
extra  cost  of  heating  soil  from  180°  to 
212°  is  less  than  2.9  cents  per  cubic 
yard1  and  therefore  economically  not 
important.  With  a  U.  C.-type  mix  it  is 
better  to  overcook  than  to  risk  incom- 
plete heating.  Other  types  of  soils  should, 
when  possible,  be  heated  to  only  180°, 
until  the  grower  finds  that  it  is  safe  to 
heat  to  212°. 

In  continuous  types  of  soil-steaming 
equipment  it  is  possible  to  control  final 
soil  temperature  to  180°  F.  In  bulk  types 
of  equipment  it  is  practicallly  impossible 
to  stop  short  of  212°  if  the  soil  is  held  in 
the  unit  for  the  full  time.  If  the  soil  is 
dumped  for  "after-cooking",  and  is 
mixed  in  the  dumping,  a  uniform  tem- 
perature of  180°  may  develop. 

'  Based  <»u  L5  per  cent  soil  moisture,  30  per 
ccul  Ileal  olVwionry,  and  natural  gas  of  1,100 
B.t.u.  per  cu.  ft.,  costing  $0,537  per  1,000  cu.  ft. 


[  140] 


OBJECTIVES  AND  DEFINITIONS 


Heat  treatment  of  soil  is  basically  a 
problem  of  transfer  of  heat  from  a 
source,  such  as  a  boiler  or  heater,  to  the 
soil  particles.  The  objectives  are  to  heat 
the  mass  uniformly  to  180°  to  212°  F, 
to  retain  this  temperature  for  30  minutes, 
and  then  to  cool  as  rapidly  as  practicable. 

Heat  transmission  through  soils,  like 
biological  phenomena  of  soils,  is  very 
complex,  difficult  to  resolve  experi- 
mentally, and  therefore  still  imperfectly 
understood.  Background  information  on 
the  dynamics  of  heat  flow  through  soil  is 
given  to  enable  more  efficient  planning 
and  practical  use  of  soil  heat  treatment. 
As  far  as  we  are  aware,  this  is  the  first 
unified  statement  of  the  principles  in- 
volved in  the  various  methods  of  heat 
treatment  of  soil. 


Heat  is  that  form  of  energy  resulting 
from  molecular  motion,  whose  intensity 
is  measured  by  the  temperature  rise  of 
the  receiving  body,  and  quantity  by  the 
B.t.u.  (British  thermal  unit)  received  by 
that  body.  The  heat  capacity  of  a  sub- 
stance is  the  quantity  (calories)  of  heat 
necessary  to  raise  the  temperature  of  1 
gram  of  it  1°  C.  This  "heat  storing" 
capacity  of  water  is  very  important  in 
the  heat  treatment  of  soils.  Steam  is  the 
vapor  phase  of  water,  which  releases  heat 
as  it  recondenses  to  water.  Heat  and 
steam  move  through  soil  in  very  different 
ways,  and  must  be  clearly  distinguished. 
Because  equipment  used  for  soil  treat- 
ment employs  either  dry  heaters  or 
steam,  the  movement  of  both  heat  and 
steam  through  the  soil  is  here  considered. 
(For  steam,  see  p.  146  through  161. ) 


TREATMENT  OF  SOIL  BY  HEAT 


Manner  of  Heat  Distribution 

The  distribution  of  heat  from  a  hot 
to  a  cooler  object  is  by  conduction,  con- 
vection, and  radiation.  The  importance 
of  each  in  soil  is  not  fully  clarified  be- 
cause of  experimental  difficulties.  Con- 
duction of  heat  is  transmission  through 
a  solid,  liquid,  or  gas  by  those  molecules 
with  greater  energy  transferring  some  of 
it,  without  any  mass  motion,  to  their 
neighbors  of  lower  energy.  The  flow  of 
heat  through  a  metal  bar  is  an  example. 
Convection  is  the  transfer  of  heat  in  a 
liquid  or  gas  by  movement  from  the 
hotter  to  the  cooler  area,  as  in  the  flow 
of  heat  from  a  floor  furnace.  Radiation 
is  the  transfer  of  heat  through  space 
from  one  body  to  another  not  in  contact 
with  it.  Much  of  the  heat  from  a  fireplace 
is  of  this  type. 


Conduction 

Heat  conduction  through  a  porous 
material  is  much  less  than  through  the 
same  material  without  pores,  because 
flow  is  greatly  reduced  by  the  contained 
air,  a  poor  conductor.  On  the  other  hand, 
conduction  is  improved  by  the  presence 
of  water  in  the  spaces.  Thus,  dry  sand- 
stone conducts  heat  about  14  times 
better  than  dry  sand,  about  7  times  better 
than  water,  and  about  175  times  better 
than  air,  at  temperatures  with  which  we 
are  concerned.  It  follows  that  heat  moves 
by  conduction  through  each  soil  particle, 
and  from  soil  particle  to  particle  through 
their  numerous  points  of  contact.  It  also 
moves  efficiently  by  conduction  from 
particle  to  particle  across  the  water  film, 
and  then  has  an  enlarged  area  of  contact. 
Although  water  improves  the  contact  be- 


[141] 


tween  particles  and  so  increases  heat 
transfer,  if  too  much  is  added  the  heat 
capacity  of  the  soil  is  so  increased  that 
there  may  be  a  decreased  rate  of  tem- 
perature rise.  Patten  (1909)  reached 
this  point  at  about  18  per  cent  water  con- 
tent (dry-weight  basis)  for  sand,  about 
10  per  cent  for  fine  sandy  loam  and  silt 
loam,  and  about  63  per  cent  for  muck 
soil  (25  per  cent  organic  matter).  This 
factor  probably  is  not  often  important  in 
heat  treatment  of  nursery  soil,  since 
moisture  is  usually  at  lower  levels.2 

Conduction  through  actual  contacts 
and  through  water  films  is  very  im- 
portant in  heat  transfer  through  soil. 
Convection  in  the  air  spaces  seems  also 
to  be  important,  though  less  clearly 
demonstrated.  Generally  considered  to  be 
less  significant  are  radiation  from  par- 
ticle to  particle  through  the  pore  air,  con- 
duction through  the  pore  air,  and  con- 
vection in  the  water  film. 

Thermal  conductivity  through  various 
types  of  mineral  soil  particles  probably 
varies  little.  Porosity,  and  therefore  par- 
ticle size  and  compaction  is,  however, 
strongly  related  to  transmission.  Thus, 
the  percentage  porosity  in  dry  soil  in- 
creases, and  the  heat  transmission  de- 
creases, in  the  following  order:  sand, 
loam,  clay,  peat.  Soil  with  particles  of 
several  sizes  tends  to  compact  readily 
owing  to  wedging  of  small  pieces  be- 
tween large  ones,  which  increases  the 
points  of  contact  and  therefore  the  con- 
duction. Radiation  across  pores  is  also 

2  In  steaming  soil,  conduction  is  relatively 
much  less  important,  since  heat  is  released  by 
condensation  of  steam  directly  on  the  soil  par- 
ticles (see  "Treatment  of  Soil  by  Steam,"  be- 
low). For  this  reason,  in  heating  with  steam 
there  is  probably  no  improvement  comparable 
to  that  observed  with  dry  heat,  from  the  addi- 
tion of  water  to  the  soil.  At  all  but  low  mois- 
ture levels,  water  actually  tends  to  reduce 
Bteaming  efficiency  by  partially  plugging  the 
pores  and  by  increasing  the  total  heat  capacity. 
This  situation  illustrates  the  necessity  of  clearly 
distinguishing  between  steam  and  heat,  already 
mentioned. 


increased,  but  convection  is  probably  de- 
creased. 

Compaction  generally  improves  move- 
ment of  heat.  This,  and  several  other 
lines  of  evidence,  indicate  that  conduc- 
tion and  radiation  are  more  important 
than  convection  in  such  movement.  On 
the  other  hand,  compaction  reduces 
movement  of  steam  and  of  chemical 
fumigants  by  diffusion  through  the 
pores.  The  addition  of  organic  matter  to 
soil  reduces  heat  transmission  by  in- 
creasing porosity.  Thus,  Newhall  (1940) 
found  that  immersion  heaters  raised  the 
temperature  of  loam  to  125.6°  F  in  3 
hours  and  147.2°  in  4  hours,  whereas 
muck  reached  only  98.6°  and  123.8°  F, 
respectively.  Morris  (19546)  provided 
a  comparison  between  the  rate  of  heat 
transmission  through  soil  by  conduc- 
tion, convection,  and  radiation,  as 
against  steam  flow.  Insufficient  heat  pene- 
trated through  1  inch  of  undisturbed  soil 
to  raise  its  temperature  to  160°  F  from 
a  steam  grid  resting  on  it,  although  dur- 
ing the  same  period  15  inches  of  loose 
soil  above  the  pipe  was  raised  above  that 
temperature.  Steam  flowed  through  the 
soil  above  and  released  its  heat,  whereas 
the  temperature  rise  in  the  soil  below 
was  probably  largely  from  heat  trans- 
mission. 

Another  aspect  of  conduction  in  the 
heating  of  soil  is  the  distribution  of  heat 
by  the  metal  container  or  cooker.  Steel 
and  iron  are  about  20  times  better  heat 
conductors  than  moist  soil,  150  times 
better  than  dry  soil,  and  75  times  better 
than  water.  Consequently,  the  soil  in  con- 
tact with  the  metal  container  will  have 
heat  indirectly  transmitted  to  it.  Metal 
liners  are  sometimes  specified  for  treat- 
ment equipment  using  immersion  heat- 
ers in  order  to  take  advantage  of  this  dis- 
tribution effect.  Steaming  of  soil  in  gal- 
lon cans  involves  very  complex  heat  ex- 
changes, since  conduction  by  the  can  to 
the  soil,  and  steam  flow  into  the  soil  sur- 
face both  occur. 


[142] 


Convection 

Movement  of  heat  through  soil  by  con- 
vection is  directly  related  to  pore  size. 
It  is  particularly  effective  in  highly 
porous  soil,  and  probably  is  important 
in  heat  transfer  through  a  U.  C.-type  soil 
mix.  Bouyoucos  (1913,  1915)  has  em- 
phasized the  importance  of  air  convec- 
tion currents  in  the  transfer  of  heat 
through  soil.  The  magnitude  of  such 
movement  through  the  pore  system  is 
shown  by  the  normal  exchange  of  soil 
carbon  dioxide  and  air  with  the  atmos- 
phere. It  has  been  computed  that  the  air 
in  some  soils  is  completely  renewed  to  a 
depth  of  about  8  to  12  inches  every  hour, 
and  that  this  is  largely  by  gas  diffusion 
through  the  pores  ( Baver,  1956).  The 
rapid  movement  of  methyl  bromide  and 
other  gases  through  soil  pores  during 
treatment  is  further  indirect  evidence  for 
the  importance  of  pores  and  convection 
currents  in  heat  transfer. 

Although  the  percentage  porosity  of  a 
soil  is  inversely  related  to  transmission 
of  heat  by  conduction  and  radiation,  it 
is  misleading  for  estimating  permeability 
to  gases.  The  number  and  size  of  the 
large  pores  are  of  great  importance  here, 
although  much  of  the  pore  space  is  not 
significantly  involved  in  convection  or 
diffusion.  Thus,  coarse  sand  ( 37.9  per 
cent  porosity)  is  about  1,000  times  more 
permeable  to  air  than  is  fine  sand  (55.5 
per  cent  porosity)  (Baver,  1956).  Simi- 
larly, a  granular  loam  was  50  to  100 
times  more  permeable  than  it  was  in  the 
powdered  state. 

Only  a  small  amount  of  clay  is  neces- 
sary in  a  sandy  soil  to  reduce  the  pore 
diameters  and  greatly  reduce  permea- 
bility. Buehrer  (1932)  found  that  addi- 
tion of  10  per  cent  clay  to  coarse  sand 
decreased  air  flow  to  about  one  fourth, 
20  per  cent  clay  to  about  one  tenth,  and 
30  per  cent  clay  to  about  one  twentieth. 
Clay  soils  may  have  50  per  cent  porositv 
and  still  be  poorly  aerated,  whereas  sand 
with  30  per  cent  porosity  may  be  well 


aerated.  He  found  that  only  part  of  the 
spaces  are  involved  in  gas  movement,  a 
considerable  part  of  them  being  blind 
alleys  or  so  small  as  to  greatly  restrict 
flow.  Only  in  soil  with  coarse  particles 
does  air  flow  reach  levels  to  be  expected 
on  the  basis  of  percentage  porosity.  It 
was  concluded  that  only  the  larger  and 
continuous  pore  systems  were  significant 
in  air  passage  through  soil.  These  would 
be  best  provided  by  coarse  or  granular 
soils  with  large  bits  of  organic  matter. 

It  would  appear  that  conduction  of 
heat  would  decrease  in  the  order  sand, 
loam,  clay,  peat,  but  that  convection  and 
diffusion  would  be  greater  in  peat  and 
sand  than  in  clay  and  loam.  When  added 
to  a  soil,  water  has  the  effect  of  reducing 
pore  size  and  permeability  to  air,  de- 
creasing convection  and  increasing  con- 
duction. This  may  contribute  to  the  slow 
heating  of  very  wet  soil  by  steam. 

Gases  increase  in  viscosity  as  the  tem- 
perature rises.  Thus,  the  time  in  minutes 
to  pull  equal  amounts  of  air  through 
soil  columns  of  moist  sand,  sandv  loam, 
clay,  and  peat,  respectively,  was  found 
by  Bouyoucos  (1915)   to  be  as  follows: 

50°  F  =  1.50,  2.00,  15.00,  and  16.40; 
86°  =  2.12,  3.37,  26.00.  and  20.40; 
122°  =  2.50,  10.35,  33.00,  and  38.40. 

The  viscosity  of  steam  likewise  in- 
creases from  125.5  at  212°  to  144.5  at 
302°  (54  lb.  pressure),  but  since  it 
moves  through  soil  at  212°  F  or  less,  this 
is  not  a  factor.  This  may  contribute  to 
the  greater  efficiency  of  heat  distribution 
through  soil  by  steam  than  by  dry  hot 
air.  Air  convection  would  decrease  as 
the  soil  became  hotter,  and  thus  decrease 
the  effectiveness  of  the  dry  type  of  heat 
transfer.  Since  the  rate  of  heat  inter- 
change between  a  gas  and  porous  object 
through  which  it  flows  is  proportional 
to  the  temperature  difference  between 
them,  this  also  causes  the  rate  of  tem- 
perature increase  to  fall  off  as  the  soil 
gets  hotter.  On  the  other  hand,  water 
convection    would    increase   with   rising 


[143] 


temperature,  and  thus  aid  heat  transfer 
in  moist  soils. 

Radiation 

Radiation  of  heat  from  the  surface  of 
a  soil  particle  occurs  in  a  straight  line 
across  the  air  space  to  another  particle. 
It  is  probably  not  very  important  in  heat 
transmission  in  soil,  but  has  not  been 
evaluated.  The  efficiency  decreases  as  the 
pore  size  increases,  and  varies  with  the 
nature  of  the  exposed  particle  surface. 
The  radiation  from  the  surface  of  the 
particle  is  improved  if  it  is  moist,  but 
does  not  attain  the  efficiency  of  water 
itself.  It  also  improves  with  increasing 
temperature. 

Tests  with  soils 

As  the  previous  discussion  has  in- 
dicated, the  three  methods  of  heat  trans- 
fer are  not  always  affected  in  the  same 
direction  by  a  given  soil  condition. 

The  conduction  of  heat  by  soil  im- 
proves as  the  percentage  porosity  and 
pore  size  decrease,  and  as  water  content 
increases.  Convection  is  favored  by  many 
large  pores,  but  decreases  in  wet  soil  and 
with  increasing  temperature.  Radiation 
is  favored  by  small  pores,  wet  soil,  and 
increasing  temperature.  It  is  thus  ap- 
parent that  only  by  trial  can  the  actual 
heat  transmission  of  a  given  soil  be  de- 
termined. The  results  of  such  tests  may 
be  instructive. 

Von  Schwarz  (1879)  placed  soils  of 
61.7°  F  in  contact  with  a  heat  source 
(140°)  and  noted  the  temperature 
reached  in  15  minutes.  Temperatures 
reached  by  dry  soils  (increasing  order 
of  transmission)  were:  peat  65.7°,  clay 
82.4°,  loam  90.0°,  sand  95.5°;  for  moist 
soils  these  were:  peat  69.6°,  clay  102.6°, 
loam  120.7°,  sand  133.2°. 

Bouyoucos  (1913)  measured  the  time 
required  for  heat  to  pass  from  a  source 
(92.3°  F)  7  inches  through  a  column  of 
soil  and  cause  the  temperature  to  start 
rising.  For  air-dry  soil  the  figures  (in- 
creasing  order   of   transmission)    were: 


peat  55.2,  loam  49.7,  clay  44.2,  and  sand 
38.0  minutes.  Similar  tests  with  soils  in 
the  field  gave  the  following  times  (in- 
creasing order  of  transmission)  at  6 
inches  from  the  heat  source:  peat  (148.6 
per  cent  water)  9  hours,  loam  (36.6  per 
cent  water)  6.5  hours,  clay  (25.9  per 
cent  water)  6  hours,  sand  (3.6  per  cent 
water)  4  hours.  In  another  test  the  time 
at  which  temperature  began  to  rise  at 
different  distances  in  similar  cores  of  dry 
quartz  sand  and  of  moist  loam,  respec- 
tively, were:  1  inch,  2  and  1  minutes;  3 
inches,  15  and  16;  4  inches,  27  and  26;  5 
inches,  34  and  35;  7  inches,  40  and  44. 
From  these  and  other  data  it  would 
appear  that  transmission  of  heat  through 
dry  soil  is  best  for  sand,  poorest  for  peat, 
and  intermediate  for  clay  and  loam  soils. 
Transmission  of  heat  is  improved  in  all 
of  these  soils  by  the  addition  of  water, 
but  the  general  order  of  transmission  is 
unchanged.  It  is  probable,  therefore,  that 
most  of  the  transmission  of  heat  is  by 
conduction,  but  that  convection  is  also 
important. 

Rate  of  Heat  Distribution 

Dry  soil 

The  rate  of  distribution  of  heat 
through  dry  soil  is  as  important  as  the 
manner  of  transmission  just  discussed. 
When  dry  soil  is  exposed  at  some  point 
to  a  constant  heat  source  such  as  an  im- 
mersion heater,  the  temperature  of  the 
soil  directly  exposed  to  the  heat  rises 
fairly  quickly.  At  a  distance  of  2  inches, 
the  temperature  will  begin  to  rise  some- 
what later,  will  rise  more  slowly,  and 
does  not  rise  as  high  as  at  the  source.  At 
4  inches  it  will  rise  later,  slower,  and  to 
a  lower  maximum  than  at  2  inches,  and 
so  on.  At  any  given  distance  there  is 
more  heat  flowing  into  the  soil  on  the 
"hot"  side  than  is  flowing  out  from  the 
cool  side.  This  differential  results  from 
the  insulating  effect  of  the  dry  soil,  the 
heat  absorbed  in  warming  the  given 
spot,   and    from   lateral   transmission   of 


I  144] 


heat  to  surrounding  particles  or  to  the 
atmosphere.  The  lateral  flow  is  im- 
portant because,  as  the  sphere  of  heat  en- 
larges, the  B.t.u.  are  being  diluted  into 
an  ever  greater  volume  of  soil  (see 
"Movement  of  Steam  through  Soil,"  be- 
low) . 

Wet  soil 

When  the  soil  is  wet,  the  advance  of 
heat  is  much  the  same,  except  that  water 
is  evaporated  near  the  source  of  heat  and 
condenses  on  the  cool  son"  farther  out. 
This  evaporation  somewhat  lowers  the 
temperature  near  the  heater,  and  in  ad- 
dition the  dry  soil  thus  produced  con- 
ducts heat  less  efficiently  than  when  wet. 
Since  the  heat  capacity  of  wet  soil  is 
higher  than  dry,  the  temperature  should 
rise  more  slowly.  These  factors  tend  to 
make  the  temperature  of  wet  soil  rise 
more  slowly  than  dry,  at  any  given  time 
or  distance  near  the  heater.  They  are 
offset,  however,  by  the  excellent  heat 
transfer  by  the  steam  formed,  and  by  the 
improved  heat  conduction  of  the  soil  at 
a  distance,  so  that  the  total  effect  of 
limited  additional  moisture  may  be  to 
slightly  increase  the  rate  of  heating.  For 
example,  we  found  that  an  electric  heater 
that  produced  120  B.t.u.  per  hour  from 


P/i  inches  of  the  tip  exposed  to  a  dry 
sandy  loam  (0.88  per  cent  water)  re- 
quired 45  minutes  to  raise  the  tempera- 
ture 32  degrees  F  at  a  2-inch  distance. 
The  same  soil  moistened  to  7.11  per  cent 
water  required  only  39  minutes  for  this 
temperature  increase.  Correcting  for  the 
heat  capacity  of  the  water,  the  times 
would  be  43.1  and  28.8  minutes,  respec- 
tively. This  again  illustrates  that  conduc- 
tion is  the  principal  means  of  heat  trans- 
mission in  soil.  When  heat  is  applied  to 
moist  soil  and  the  water  at  a  given  point 
has  been  evaporated,  the  temperature 
rise  follows  that  described  for  dry  soil. 
Thus,  heat  applied  to  a  moist  soil  is 
intermediate  between  dry  heat  and  steam, 
more  nearly  approximating  the  former. 

The   steady  and   the 
unsteady  states 

The  above  conditions  apply  when  heat 
is  advancing  through  dry  or  wet  soil — 
the  so-called  unsteady  state.  If  the  heat 
has  been  constantly  applied  for  a  suf- 
ficiently long  time,  and  the  surrounding 
conditions  are  also  constant,  each  point 
in  the  soil  remains  at  a  given  tempera- 
ture— the  so-called  steady  state  (fig.  72) . 
At  that  time  the  temperature  decreases 
uniformly  with  distance  from  the  source. 


Temperature 


Steady  state 


Distance  from  heat  source 

Fig.  72.  Temperature  gradients  with  distance  from  a  heat  source,  in  the  advancing  (unsteady) 
and  steady  states.  (Based  on  Patten,  1909.) 


[145] 


For  a  given  heat  input  and  surrounding 
conditions  affecting  temperature  loss, 
there  is  a  distance  beyond  which  there  is 
insufficient  heat  transmitted  for  the  soil 
to  reach  180°  or  212°  F.  In  practical 
heat  treatment  of  soil  the  steady  state  is 
almost  never  reached,  and  the  heat  dis- 
tribution through  the  soil  is  improved 
by  increasing  the  number  of  heat  sources 
per  volume  of  soil. 


One  of  the  worst  disadvantages  of  a 
dry  source  of  heat  in  soil  treatment  is 
that  intensity  (temperature)  is  high, 
quantity  (B.t.u.)  is  small,  and  distribu- 
tion is  poor.  Steam,  by  contrast,  imparts 
a  large  quantity  of  heat  at  low  intensity 
(212°  F)  and  flows  through  the  soil  to 
the  cold  areas.  One  of  the  principal  ad- 
vantages of  steam  is  that  the  B.t.u.  are 
released  at  the  point  to  be  heated. 


TREATMENT  OF  SOIL  BY  HOT  WATER 


Heat  distribution  by  hot  water  in- 
volves different  factors  than  those  out- 
lined above.  The  water  is  applied  to  the 
surface  and  flows  by  gravity  through  the 
soil  pores,  heat  being  transferred  to  the 
soil  particles  by  conduction.  The  first 
water  displaces  the  air  and  fills  the  pores, 
additional  hot  water  pushes  the  cooled 
water  downward.  The  temperature  de- 
creases from  top  to  bottom,  and  the  low- 
est level  approximates  the  temperature 
of  the  draining  water.  There  is  some 
lateral  spread,  but  not  enough  to  heat 
very  far.  Because  even  boiling  water  has 
only  212  B.t.u.  per  pound,  minus  the 
existing  soil  temperature,  the  tempera- 
ture rise  is  very  slow  and  a  great  deal  of 


water  is  required.  Only  142  B.t.u.  are  re- 
leased, for  example,  in  cooling  1  pound 
of  water  from  212°  to  70°  F,  whereas 
1,112  B.t.u.  are  released  in  similarly 
cooling  1  pound  of  steam.  Thus,  with 
soil  at  70°,  7.8  times  more  moisture 
must  be  added  with  boiling  water  than 
with  steam. 

Because  of  the  effectiveness  of  hot 
water  in  leaching  salts  from  soil,  flood- 
ing the  propagating  bench  with  it  before 
use  is  an  excellent  practice.  If  the  messi- 
ness  of  flooding  with  sufficient  water  to 
raise  the  temperature  to  180°  F  makes 
heating  in  this  way  impracticable,  the 
leaching  may  well  be  followed  with  a 
steam  treatment  to  free  the  soil  of  patho- 
gens. 


TREATMENT  OF  SOIL  BY  STEAM 


Condensation   of   Steam 
in   Soil 

As  the  steam  moves  into  the  pores  of  a 
soil  it  mingles  with  the  air  held  there, 
the  ratio  of  steam  to  air  rising  with  in- 
creased time.  The  condensation  of  such 
water  vapor  is  determined  by  the  tem- 
perature difn  rential  between  the  vapor 
and  the  soil  particles,  and  by  the  ratio 
of  steam  to  air.  as  pointed  out  by  Hoare 
(1953),  Morris  (1954a),  and  Bunt 
(1955). 


The  dew  point  is  the  highest  tempera- 
ture at  which  the  quantity  of  water  vapor 
in  the  air  is  sufficient  to  saturate  it  and 
cause  condensation.  The  lower  the  con- 
centration of  water  vapor  (relative  hu- 
midity), the  lower  the  temperature  at 
which  condensation  occurs.  For  ex- 
ample, at  70°  F  all  water  vapor  in  excess 
of  1  pound  to  each  63.3  pounds  of  dry 
air  will  condense  (fig.  73);  at  100°  the 
critical  ratio  is  1:23.2;  at  130°,  1:9.0; 
at    190°,  1:0.9;  at  211°,  1:0.03;  and  at 


|    I  10 


Lbs.  dry  air 

Fig.  73.  Maximum  pounds  of  dry  air  per  pound  of  water  vapor  at  which  condensation  occurs 
at  various  temperatures.  The  ratio  of  steam  to  air  at  a  given  temperature  may  be  determined 
by  reading  down  from  the  intersection  of  the  temperature  and  the  condensation  lines.  Thus,  the 
ratio  for  90°F  is  1:32.  (Calculated  from  data  of  Zimmerman  and  Lavine,  1945.) 


212°,  1:0  (that  is,  pure  steam).  This  is 
true  whether  it  refers  to  condensation  of 
humidity  in  the  glasshouse  or  of  steam 
in  a  bench  of  soil.  These  facts  strongly 
affect  the  manner  in  which  steam  heats 
soil. 

Relation   of  steam/air 
ratio   to   condensation 

When  steam  is  released  into  soil  of  70° 
F,  it  expands  if  it  has  been  under  pres- 
sure in  the  pipe,  and  drops  to  approxi- 
mately atmospheric  pressure  and  soon 
to  212°.  It  condenses  quickly  on  the 
cool  particles  at  its  point  of  entry  be- 
cause of  the  low  steam/air  ratio  (1 :63.3  l 
at  that  temperature.  As  the  temperature 
of  the  soil  and  soil  air  rises  from  the 
heat  released  by  condensation,  an  ever 
richer  mixture  of  steam  and  air  must  be 


reached  if  condensation  is  to  continue 
(for  example,  at  90°,  this  is  1:32.1). 
On  a  given  soil  particle  steam  would  con- 
tinue to  condense  until  the  temperature 
became  too  high  for  the  existing  steam/ 
air  ratio.  Since,  however,  steam  is  con- 
tinuously released,  and  some  air  is 
pushed  ahead  of  the  incoming  steam,  the 
ratio  rises  steadilv  until  all  air  is  dis- 
placed. Condensation  is,  therefore,  con- 
tinuous until  212°  is  reached.  The  steam 
then  flows  on  to  the  cooler  advancing 
zone. 

Incidentally,  this  principle  explains 
why  an  autoclave  type  of  soil  cooker  op- 
erating at  15  pounds'  pressure  must  have 
air  displaced  by  steam  (through  operat- 
ing with  the  exhaust  or  door  open  for  a 
time)  before  the  temperature  can  reach 
the  expected  249.8°  F. 


[147] 


Treating   soil   with 
steam-air  mixtures 

From  the  above  discussion  on  steam: 
air  ratio  it  is  apparent  that  the  steam 
temperature  may  be  reduced  by  injecting 
and  mixing  air  into  the  line  near  the  out- 
put. Recent  investigations  by  Morris 
(1954a)  and  Bunt  (1955)  in  England 
have  ultilized  this  principle  to  heat  soil 
with  steam  to  a  final  temperature  below 
212°  F.  By  employing  a  venturi  tube  in 
a  steam  line  carrying  40  to  50  pounds' 
pressure,  enough  air  has  been  drawn  in 
to  reduce  the  steam  temperature  to  160°. 
If  low-pressure  steam  is  used,  however, 
a  pump  is  necessary  to  inject  air  into  the 
flowing  steam.  When  180°  steam  is  in- 
jected into  soil,  the  steam  condenses  on 
the  particles  as  before.  The  heated  air  is 
pushed  ahead,  forming  a  band  3  to  4 
inches  wide,  the  temperature  of  which 
remains  between  that  of  the  heated  and 
unheated  soil  for  2  to  4  minutes.  The 
zone  of  condensation  of  212°  steam  in 
soil  may  be  1  inch  wide  or  less  (see  be- 
low) and  remain  for  only  10  to  20 
seconds.  Savings  of  up  to  15  per  cent  in 
fuel  have  been  reported.  As  already  men- 
tioned in  this  section,  the  calculated  sav- 
ing from  heating  soil  to  180°  instead  of 
212°  is  less  than  2.9  cents  per  cubic 
yard. 

The  method  is  not  yet  ready  for  com- 
mercial application.  When  it  is  available 
it  can  be  attached  to  equipment  types  1 
to  8,  18  to  23,  and  26  to  28  (Sec.  10). 
Its  application  to  equipment  with  a  mov- 
ing soil  mass,  or  to  pressure  chambers,  is 
more  doubtful. 

Treating  soil  with  a  steam-air- 
chemical  mixture 

The  combination  of  a  volatile  fungi- 
ride  with  a  low  temperature  steam-air 
mixture  instead  of  with  212°  F  steam 
(Sec.  10,  type  26)  would  appear  to  offer 
possibilities  for  effective  cheap  treat- 
ment of  soil.  Since  the  velocity  of  chemi- 
cal reactions  increases  two  to  three  fold 


CAUTION: 

Many 

of 

the  chemicals 

mentioned 

in  this 

manual  are 

poi- 

sonous   and 

may 

be 

harmful. 

The 

user  should 

carefu 

lly 

Follow  the 

pre- 

cautions  on 

the  1 

abe 

Is  of  the 

con- 

tainers. 

with  each  18  degrees  F  rise  in  tempera- 
ture, it  is  evident  why  the  combination  of 
heat  and  chemicals  is  substantially 
cheaper  than  steam  alone.  Beachley 
(1937)  found  that  the  combination  of 
formaldehyde  with  212°  steam  reduced 
by  one  third  the  time  necessary  for  in- 
verted pan  treatment  with  steam,  and 
the  cost  by  one  fifth;  it  was  one  third 
cheaper  than  a  formaldehyde  drench. 

Soil  conditions  affecting 
steam  penetration 

The  air  in  the  labyrinth  of  pores  is  ex- 
pelled at  a  rate  proportional  to  the 
volume  of  steam  injected;  the  rates  are 
not  equal  because  of  the  volume  reduc- 
tion from  condensation  of  the  steam. 
When  large  volumes  of  steam  are  used, 
the  air  is  displaced  so  rapidly  by  the 
mass  flow  that  it  may  not  be  appreciably 
heated.  With  the  usual  smaller  steam 
quantity,  the  air  is  also  heated  as  it  is 
pushed  ahead,  and  transfers  its  heat  in 
turn  to  cooler  soil  particles.  Since  the 
heat  capacity  of  air  is  about  half  that  of 
steam  (see  Appendix),  the  quantity  of 
heat  transferred  in  this  way  by  air  is  not 
large.  It  may  contribute,  however,  to  the 
gradual  temperature  rise  at  a  given  point 
in  soil  heated  by  inefficiently  small 
volumes  of  steam  (figs.  75  and  76) .  Cer- 
tainly this  air  movement  would  reinforce 
convection  and  diffusion  in  the  continu- 
ous soil  pores.  When  air  is  injected  into 
the  steam  (see  above)  the  importance  of 
heat  transmission  by  the  air  is  relatively 
greater  than  when  steam  is  used  alone. 

As  already  indicated,  much  of  the  soil 
pore  space  is  not  involved  in  movement 
of  gases,  there  being  many  pockets  and 
plugged    or    partially    blocked   channels. 


[148] 


The  steam  flows  freely  along  the  open 
channels,  condensing  on  the  adjacent 
particles.  The  heat  thus  released  is  trans- 
mitted to  the  surrounding  particles  by 
conduction,  radiation,  and  convection 
within  the  sealed  pores  (see  "Treatment 
of  Soil  by  Heat — Manner  of  Heat  Dis- 
tribution," above). 

A  clod  of  soil  may  be  surrounded  and 
by-passed  by  steam  before  it  has  been 
heated  to  212°  F.  In  a  soil  consisting  of 
uniform  lumps,  the  thickness  of  the  layer 
being  heated  at  any  moment  depends  on 
the  rate  of  heat  supply  and  absorption. 
If  both  the  lumps  and  the  steam  volume 
are  large,  the  vapor  will  be  lost  through 
the  surface  during  much  of  the  opera- 
tion. The  same  might  apply  to  soil  con- 
taining many  large  pieces  of  organic 
matter,  or  having  cracks  either  in  it  or 
between  it  and  the  container.  In  such 
cases  the  width  of  the  zone  of  condensa- 
tion is  determined  by  the  depth  of  soil 
treated  (see  also  "Soil  structure,"  be- 
low). 

Steam  penetration  of  dry  soil  may 
tend  to  be  slower  than  for  moist  soil, 
owing  to  the  greater  tendency  to  com- 
paction and  smaller  pores  of  the  former, 
as  well  as  reduced  conductivity  from  low 
moisture  content.  Water  apparently  af- 
fects the  flow  of  gases  through  soil 
largely  by  its  effect  on  pore  size  and  soil 
structure. 

As  mentioned  earlier,  when  soil  is  too 
wet,  the  high  heat  capacity  of  water  re- 
tards heating.  Because  of  this,  by  the 
time  the  water  is  heated  to  212°  F,  so 
much  steam  will  have  condensed  that  the 
water-holding  capacity  of  the  soil  will  be 
exceeded  and  water  will  drain  from  the 
bottom  of  the  bench  (Sec.  8).  This  may 
produce  a  broad  temperature  gradient  in 
the  lower  soil  levels. 

Movement  of  Steam  through  Soil 

Steam  moves  through  the  air  spaces 
of  the  soil  in  the  same  way  as  any  other 
gas  (see  "Manner  of  Heat  Distribution 
— Convection,"    above).    This   certainly 


involves  diffusion,  and  probably  also 
eddy  currents,  through  the  continuous 
pore  system  of  the  soil.  Several  facts  in- 
dicate that  steam  does  move  through 
soil  in  this  manner.  It  moves  very  slowly 
through  compact  soil  (for  example,  into 
clods  and  subsoil — see  "Soil  structure," 
below).  It  moves  more  rapidly  through 
a  soil  to  which  organic  matter  is  added, 
or  which  is  lumpy.  Thus,  we  found  that 
at  5.54  per  cent  moisture  and  with  a 
single  steam  input  of  3.5  pounds  per 
hour,  a  sandy  loam  required  33  minutes 
for  the  temperature  to  rise  142  degrees  F 
at  a  distance  of  5  inches,  whereas  U.  C. 
mix  C  (50  per  cent  peat)  required  but 
18.5  minutes.  Within  limits,  the  larger 
and  more  numerous  the  pores,  and  the 
more  continuous  the  system  they  form, 
the  better  the  penetration  of  steam.  The 
pores,  however,  must  restrict  the  flow 
enough  that  steam  does  not  "blow  out" 
through  the  surface  before  condensing. 
When  all  of  the  soil  has  been  heated  to 
212°  the  steam  does  not  condense,  but 
seeps  out  of  the  exposed  surface;  this  is 
visible  evidence  of  the  ready  flow  of 
steam  through  soil.  Pressure  flow  is 
probably  not  involved  in  such  move- 
ment, as  steam  has  little  or  no  more  than 
atmospheric  pressure  in  the  soil. 

With  a  small  flow  of  steam 

The  way  in  which  steam  moves 
through  soil,  and  the  rate  of  temperature 
rise  produced  by  it,  are  both  strongly 
affected  by  the  volume  of  steam  used, 
and  the  distance  from  the  point  of  in- 
jection (see  below,  and  figs.  74  and  76) . 
When  a  small  flow  of  steam  is  used,  the 
mixture  of  steam  and  air  in  the  soil  pores 
at  a  given  point  becomes  progressively 
richer  with  increasing  time  and  with 
proximity  to  the  input,  as  already  ex- 
plained. The  pores  of  the  soil  around  the 
input  are  soon  filled  with  pure  steam, 
and  the  temperature  reaches  212°  F.  At 
the  moment  steam  was  injected  into  soil, 
the  temperature  in  one  of  our  tests  was 
188.4°    (a   steam  air    ratio    of    1:1)    2 


[  149] 


Wide 


o 

E 

o 

d) 

Q) 

c 
o 

to 

N 

D) 

«-»- 

c 

o 

t/5 

_c 

c 

TD 

~o 

£ 

c 
o 

Narrow 


Efficiency 
Low 


High 


Low 


Steam  Flow  Rate 
Ratio  of  Steam  to  Air 


High 


Low 


Fig.  74.   Diagram  showing  the  relations  of  steam  flow  rate  to  the  steam:  air  ratio,  width  of 
zone  of  condensation  in  soil,  and  efficiency.  See  p.  152  through  154  for  explanation. 


inches  away,  at  2%  inches  it  was  148.2° 
(a  ratio  of  1:5),  at  3%  inches  105.4° 
(1:20),  at  4  inches  76.8°  (1:50),  and  at 
5  inches  71.5°  (1 :60) .  Ten  minutes  later 
the  same  relations  would  still  exist,  but 
at  greater  distances  from  the  input.  Ap- 
proximately the  same  situation  exists  for 
higher  steam  injection  rates  at  points  in 
the  soil  distant  from  the  input  (fig.  76). 
That  the  ratio  of  steam  to  air  is  the 
principal  factor  involved  in  the  wide 
condensation  zone  at  low  steam  flow 
rates  is  shown  by  data  of  the  type 
graphed  in  figure  75.  In  these  tests  with 
U.  C.  mix  C  (50  per  cent  peat)  we  meas- 
ured the  temperatures  at  several  dis- 
tances from  the  steam  input,  and  im- 
mediately collected  a  large  soil  sample 
from  around  each  thermocouple  to  de- 
termine, by  oven-drying,  the  condensed 
steam  they  contained.  The  temperature 
curve  and  the  percentage  of  moisture 
both  decrease  with  distance  from  the 
steam  input.  The  faster  temperature  rise 
than  would  be  expected  from  the  amount 
of  condensed  steam  may  have  been 
partly  due  to  experimental  limitations. 
The  suggestion  is  strong,  however,  that 
there  was  some  conduction  of  heat  in  ad- 
dition to  t lu-  steam  flow.  It  is  largely  to 
preserve  efficiency,  by  reducing  the  dis- 


tance of  steam  travel,  that  perforated 
pipes  and  steam  rakes  are  placed  at  in- 
tervals in  a  stationary  soil  mass  in  most 
types  of  equipment  (see  below  and  Sec. 
10),  or  that  soil  is  broken  into  small 
volumes  by  being  placed  in  flats  or  pots. 

It  is  apparent  from  these  facts  that 
there  is  a  wide  zone  (several  inches)  of 
condensing  steam  in  the  soil  under  con- 
ditions of  reduced  steam  flow,  both  close 
to  and  distant  from  the  input.  The  width 
of  this  zone  in  a  given  soil  at  a  certain 
distance  from  the  steam  input  narrows 
as  the  steam  flow  rate  is  increased. 

The  importance  of  heat  transfer  by 
conduction,  convection,  and  radiation 
after  condensation,  in  the  case  of  steam 
with  a  small  flow  rate,  is  not  known.  It 
would  be  expected,  however,  that  they 
would  become  more  important  as  the  in- 
put flow  reached  very  low  levels.  When  a 
point  is  reached  at  which  the  heat  lost 
from  exposed  soil  surfaces  balances  that 
introduced  by  the  steam,  no  further  tem- 
perature rise  will  occur  (the  steady 
state) . 

Transmission  through  soil  of  the  heat 
released  by  a  small  flow  of  steam  would 
fall  off  rapidly  with  distance.  This  is  be- 
cause of  the  B.t.u.  required  to  heat  a 
given    point    of    soil,    and    because    of 


1 1  ™  ] 


lateral  transfer  of  heat  to  surrounding 
cool  particles.  Because  of  the  increasing 
volume  of  the  spheroid  of  steam  (see 
below) ,  the  number  of  cool  particles  and 
the  number  of  the  plugged  pores  not 
penetrated  by  steam  increases  in  each 
successive  layer.  Thus,  there  is  the 
double  effect  of  the  increasing  volume  of 
soil  for  condensing  steam,  and  the  de- 
creasing rate  of  transmission  of  heat 
with  distance. 

These  two  effects  are  well  illustrated 
by  MacLean's  (1930)  data  on  the  heat 
penetration  of  green  pine  logs  in  a 
steam  autoclave.  In  his  tests,  heat  but  not 


steam  penetrated  the  timbers.  The  tem- 
perature at  a  given  depth  from  the  sur- 
face decreased  as  the  log  size  increased, 
although  other  conditions  were  uniform; 
the  increasing  volume  thus  presented 
corresponds  to  the  expanding  spheroid 
of  soil  discussed  below.  The  temperature 
also  decreased  progressively  toward  the 
center  of  each  log,  despite  the  fact  that 
the  volume  became  progressively  less. 
Thus  the  temperature  decreased  whether 
volume  increased  or  decreased.  This  in- 
dicates that  the  decreasing  transfer  of 
heat  with  distance  through  a  solid  is 
more  important  in  causing  the  lowering 


12  3  4  5 

Distance  (in.)  from  steam  input 

Fig.  75.  Relation  of  soil  temperature  and  percentage  moisture  (condensed  steam)  with  increas- 
ing distance  from  the  steam  input.  U.  C.-type  soil  mix  C,  moisture-free  at  beginning  of  test, 
injected  with  steam  of  0.6  to  1.1  lb.  per  hr.  for  12  to  20  min.,  when  data  were  taken.  Average 
of  three  series.  The  temperature  of  the  soil  (broken  line)  and  the  per  cent  moisture  (solid  line)  it 
contains  rise  together.  Both  also  decrease  with  distance  from  the  steam  input.  This  is  because 
the  amount  of  air  in  the  steam  increases  with  distance  from  the  input. 


[151] 


temperatures  than  is  the  increase  of 
volume  from  the  enlarging  sphere  of 
heat  penetration.  From  this  considera- 
tion, the  rate  of  temperature  rise  in  soil 
from  transmitted  heat  may  be  described 
as  decreasing  rapidly  with  distance  be- 
cause of  resistance  to  heat  flow,  and  this 
decrease  is  reinforced  by  the  enlarging 
volume  of  the  spheroid. 

There  is  a  further  tendency  for  steam 
of  small  flow  rate  to  contain  more  en- 
trained water  than  that  of  high  rates, 
because  of  greater  relative  condensation 
in  the  lines.  Low  input  rates  often  tend 
because  of  this  to  diminish  the  pore  size 
in  the  soil  and  to  further  restrict  steam 
movement,  particularly  when  the  perfo- 
rated pipes  are  on  the  bottom  of  a  tight 
bin  of  soil  with  poor  drainage. 

With  intermediate  and 
high  flow  rates 

Morris  (1954a,  19546)  has  carefully 
studied  the  movement  through  soil  of 
steam  at  intermediate  and  high  flow 
rates.  It  moves  rapidly  through  porous 
or  lumpy  soil  or  when  a  large  volume  of 
steam  is  used,  and  more  slowly  in  uni- 
formly fine  soil  or  with  lower  steam 
volume.  There  is  a  maximum  rate  at 
which  a  given  soil  can  condense  steam. 
Beyond  this  rate  the  condensation  front 
becomes  wider  as  the  steam  rushes  by 
particles  without  condensing,  the  steam 
"blows  out"  through  the  surface,  and  ef- 
ficiency is  very  low.  At  a  somewhat 
lower  range  the  steam  condenses  in  a 
narrow  zone  whose  thickness  decreases 
with  the  steam  quantity.  Morris  and 
Winspear  (1957)  were  able,  by  using  a 
quick-acting  thermocouple  recorder,  to 
demonstrate  this  advancing  condensa- 
tion zone  in  soil.  In  one  test,  a  point  21/i> 
inches  above  the  steam  source  increased 
from  38.7°  F  at  2.12  minutes  to  212° 
0.75  minute  later,  a  point  7%  inches 
above  the  source  increased  from  38.3° 
at  9.25  minutes  to  212°  1  minute  later, 
and  a  poinl  L'3  inches  above  increased 
from  39.0°  at  17.38  minutes  to  212°  0.5 


minute  later.  Thus,  the  212°  front  moved 
10%  inches  in  15.01  minutes,  or  about 
0.7  inch  per  minute.  The  front  passed 
the  three  points  in  0.5,  0.75,  and  0.25 
minute,  respectively  (an  average  of  0.5 
minute),  and  from  this  the  thickness  of 
the  front  was  computed  to  be  only  0.35 
inch.  They  concluded  that,  for  efficient 
use  of  steam,  the  thickness  of  this  zone 
should  be  1  inch  or  less,  and  that  the 
total  surface  area  of  the  soil  particles 
in  the  zone  must  be  sufficient  to  condense 
the  steam  supplied.  In  this  range  the  ef- 
ficiency is  high. 

It  is  apparent  from  the  above  facts 
that  at  very  low  steam  flow  rates  the 
zone  of  condensation  in  soil  is  broad, 
there  is  much  mixing  of  the  soil  air  with 
steam,  and  efficiency  is  low.  As  the  flow 
rate  increases,  the  condensation  zone 
narrows,  the  amount  of  air  mixed  with 
the  steam  lessens,  and  efficiency  rises. 
Finally  a  flow  volume  is  reached  at  which 
the  condensation  zone  is  at  its  narrowest, 
there  is  very  little  mixing  of  air  with  the 
steam,  and  efficiency  is  at  its  highest.  An 
opposing  factor  begins  to  operate  at 
about  this  point:  as  the  maximum  con- 
densing power  of  the  given  soil  is 
reached,  there  is  an  increasing  tendency 
for  steam  to  "blow  out"  of  the  soil  sur- 
face. Once  the  condensing  power  of  the 
soil  is  exceeded,  the  efficiency  falls 
rapidly,  because  the  mass  flow  of  steam 
rushes  by  the  particles  without  condens- 
ing. In  this  situation  there  is  insignificant 
mixing  of  air  with  the  steam,  the  air  be- 
ing pushed  out  ahead  of  the  steam,  and 
the  condensation  front  again  widens. 
Thus,  as  the  steam  flow  rate  increases 
from  very  low  to  high  the  width  of  the 
condensation  zone  passes  from  very  wide 
to  narrow,  and  again  to  very  wide,  as 
different  factors  come  into  play.  This  is 
schematically  shown  in  figure  74.  Be- 
cause of  the  multiplicity  of  factors  in- 
volved, it  is  not  possible  to  define  the 
limits  of  each  of  these  levels  of  steam 
efficiency  accurately.  It  is  obvious,  how- 
ever, that  there  is  a  lower,  as  well  as  an 


[152] 


upper  limit  for  efficient  use  of  steam. 
Best  commercial  practice  is  to  use  a 
steam  flow  just  below  the  rate  that  gives 
surface  "blow  out." 

Expanding  spheroids  of  steam 

Morris  (19546)  investigated  the 
proper  spacing  of  steam  outlets  in  soil. 
"It  is  a  safe  rule  ....  that  the  sterilising 
effect  can  reach  to  1V2  times  the  depth 
of  the  pipes  ....  and  the  steam  should 
be  injected  at  not  less  than  %  of  the  total 
depth  ....  The  space  between  the  hori- 
zontal pipes  should  not  exceed  the  depth 
of  the  holes  by  more  than  25%  and  the 
spacing  ....  along  the  pipe  should  be 
about  equal  to  the  hole  depth." 

From  these  specifications,  the  findings 
of  Bunt  (1954-55),  and  our  observa- 
tions, it  is  concluded  that  steam  moves 
out  from  an  orifice  into  soil  as  a  sphe- 
roid with  an  elongated  top,  the  margins 
laterally  and  downward  being  approxi- 
mately half  that  of  the  upward  limit. 
This  is  in  accordance  with  the  tendency 
of  heat  to  rise.  The  lateral  movement 
may  exceed  the  downward  flow  by  as 
much  as  one  fourth.  This  is  the  status 
during  the  advancing  state.  When  two 
expanding  spheroids  overlap,  the  steam 
probably  flows  toward  the  unheated 
corners,  because  of  the  pressure  reduc- 
tion there  caused  by  condensation  of  the 
steam,  and  the  fact  that  all  of  the  pore 
space  at  the  point  of  overlapping  would 
already  be  filled  with  steam.  When  the 
soil  mass  including  all  of  the  corners 
and  spaces  between  spheroids  is  heated, 
each  injection  point  will  have  heated  a 
rectangular  volume  whose  dimensions 
will  be  in  the  approximate  ratio:  dis- 
tance from  outlet  to  soil  surface  (or 
lower  limit  of  next  rectangle  above,  in  a 
multilayer  pipe  grid)  =  1.0;  distance 
from  outlet  to  bottom  (or  to  upper  limit 
of  next  rectangle  below)  =  0.5;  distance 
to  lateral  sides  (or  to  lateral  limit  of  ad- 
jacent rectangles)  =  0.5  to  0.625.  This 
relation  is  shown  diagrammatically  in 
figure  71. 


The  farther  out  the  steam  flows  from 
the  orifice  the  greater  is  the  volume  of 
soil  into  which  it  passes.  The  approxi- 
mate volume  of  the  spheroid  when  the 
steam  has  advanced  1  inch  horizontally 
from  the  input  is  6  cubic  inches,  at  2 
inches  it  is  50,  at  3  inches  170,  at  4 
inches  402,  at  5  inches  785,  and  at  6 
inches  1,357  cubic  inches.  Because  of 
this  sharp  increase  in  the  volume  of  soil 
into  which  the  steam  passes  in  its  out- 
ward flow,  as  well  as  the  factors  already 
mentioned,  the  rate  of  extension  of  the 
spheroid  of  steam  rapidly  falls  off  with 
distance  from  the  source.  It  would  thus 
take  about  215  times  as  long  to  heat  a 
6-inch  as  it  would  a  1-inch  spheroid  of 
soil. 

Increasing  the  distance  from  a  fixed 
steam  flow  has  the  same  retarding  effect 
on  temperature  rise  as  does  decreasing 
the  rate  of  steam  flow  at  a  fixed  distance 
(fig.  76),  and  for  the  same  reasons. 
Thus,  in  our  tests  a  U.  C.  soil  mix  C  (50 
per  cent  peat)  with  5.54  per  cent  mois- 
ture reached  212°  F  in  8  minutes  at  a 
point  6  inches  from  the  input  of  6.7 
pounds  of  steam  per  hour,  but  only  3 
inches  from  the  input  of  2.18  pounds 
per  hour.  Distance  and  steam  volume  are 
to  this  extent  mutually  compensating. 

Efficient  Rates  of  Steam  Flow 

Steam  may  be  efficiently  used  over  a 
considerable  range  of  flow  rates  and 
penetration  distances  into  soil.  Neither 
the  steam  volume  nor  the  lateral  distance 
should  be  so  great  that  steam  escapes 
from  the  surface  before  the  mass  is 
heated.  At  the  other  extreme,  the  steam 
volume  should  not  be  so  small,  nor  the 
distance  between  outlets  so  great,  that 
the  heat  losses  from  the  surface  and 
sides  offset  the  input,  and  necessitate 
extended  steaming  periods.  In  other 
words,  the  condensation  zones  should  be 
neither  too  wide  nor  too  narrow,  ap- 
proximating 1  inch  or  slightlv  less. 
Efficient  steaming  thus  requires  that  the 


[153] 


°F 
210" 

200- 
190- 
180- 

170- 
160- 
150- 
140- 

130- 

120- 

110- 

100- 

90- 

80^ 

70 


U.  C.  Mix  6.7  lbs.  per  hour 
2  in. 


3  in.  A  . 


4  in. 


5  in  ,  . 
6  in 


1 1 1 1 1 

U.  C.  Mix  1.45  lbs.  per  hour 


'5  in. 


/  6  in. 


0     10     20     30    40    50    60 
Time  (min.) 


U.  C.  Mix  3.5  lbs.  per  hour 

/ 
/ 

k 


1 — i — i — i — i — i 

U.  C.  Mix  0.93  lb.  per  hour 


U.C  Mix  2.18  lbs.  per  hour 
/6 


Sandy  loam  3.5  lbs.  per  hour 


0     10     20    30    40    50    60 
Time  (min.) 


—1 1 

0     10     20    30    40    50    60 

Time  (min.) 


Fig.  76.  Temperature  gradients  of  U.  C.-type  soil  mix  C  (5.54  per  cent  water  content)  at  five 
distances  from  the  steam  input  and  at  five  steam  flow  rates.  A  chart  for  one  comparable  series 
with  sandy  loam  illustrates  the  effect  of  organic  matter  on  steam  penetration.  See  p.  153  and 
155  for  explanation. 


[154] 


temperature  rise  at  any  point  should  be 
rapid,  once  it  has  started  (fig.  74). 

In  tests  at  Los  Angeles,  U.  C.  soil  mix 
C  (50  per  cent  peat)  with  5.54  per  cent 
moisture  was  injected  at  a  single  point 
with  varying  quantities  of  steam.  The 
times  for  the  temperature  to  start  to  rise 
from  70°  F,  and  to  reach  212°  there- 
after, respectively,  5  inches  from  the  in- 
put were  as  follows: 

6.7  pounds  steam  per  hour,  3  and 
4  minutes ; 

3.5  pounds,  10.5  and  6.5  minutes; 

2.18  pounds,  10.9  and  21.7  minutes; 

1.45  pounds,  22.5  and  52.5  minutes. 
At  the  indicated  rates  of  flow  for  the 
total  time  to  heat  each  5-inch  spheroid, 
the  B.t.u.  required  would  be  754,  962, 
1,149,  and  1,758,  respectively.  This 
series  shows  an  increasing  efficiency  with 
increasing  steam  flow  into  the  spheroid. 
Probably  heat  was  transmitted  to  the 
surrounding  soil  by  conduction,  convec- 
tion, and  radiation,  and  this  played  an 
increasing  role  as  steam  flow  was  de- 
creased. Since  the  upper  limit  of  the 
condensing  capacity  of  the  soil  was  not 
exceeded,  there  was  no  falling  off  at  the 
higher  volumes. 

Bunt  (1954-55)  also  found,  for  soil 
in  bins,  a  decrease  in  the  amount  of 
steam  per  cubic  foot  of  soil,  and  in  time, 
to  reach  212°,  as  the  steam  flow  was  in- 
creased. When  a  given  volume  of  soil 
was  treated  in  34  minutes,  7.35  pounds 
of  steam  per  cubic  foot  was  required; 
when  the  flow  was  increased  so  that  only 
8  minutes  were  required.  5.40  pounds 
was  used.  He  attributed  the  inefficiency 
at  low  flow  rates  to  heat  losses  from  the 
soil  surfaces. 

In  our  test  the  lower  limit  of  practical 
efficiency  probably  was  the  2.18  pounds 
per  hour  flow  per  orifice.  At  the  1.45- 
pound  rate,  the  temperature  rise  was  so 
slow  (52.5  min.)  that  heat  transmission 
by  conduction,  convection,  and  radiation 
probably  came  into  play.  One  of  the 
principal   advantages  of  steam    (the  re- 


lease of  B.t.u.  at  the  point  to  be  heated) 
was,  therefore,  diminished. 

Bunt  (1954-55)  found,  on  the  other 
hand,  that  thermal  efficiency  was  greater 
with  moderate  rather  than  with  large 
steam  flow  rates  for  soil  in  ground  beds. 
This  is  because  there  is  less  opportunity 
for  heat  loss  from  exposed  surfaces  than 
there  is  in  benches  or  bins. 

Steam  "blow-out" 

The  upper  limit  of  steam  flow  rate  for 
a  given  soil  is  recognized  by  the  ten- 
dency to  "blow  out"  from  the  surface 
before  most  of  the  soil  is  heated  to  212° 
F.  Obviously  this  rate  should  not  be 
exceeded  for  efficient  operation.  It  would 
appear,  furthermore,  that  the  flow  rate 
should  not  fall  far  below  this  level  for 
maximum  efficiency.  The  time  for  the 
temperature  to  rise  to  212°  F  several 
inches  from  the  steam  input  is  a  useful 
measure  of  this  range.  In  other  words, 
the  steam  flow  should  utilize  fully,  but 
not  exceed,  the  condensation  capacity  of 
the  given  soil.  Since  this  "balanced 
steaming"  is  determined  by  so  many 
factors,  it  is  best  found  by  trial  for  the 
given  soil  and  conditions. 

The  tendency  for  steam  to  "blow  out" 
before  the  soil  is  treated  may  be  mini- 
mized by:  (1)  reducing  the  rate  of  steam 
flow;  (2)  reducing  the  steam  pressure  at 
the  point  of  injection  into  soil;  (3)  hav- 
ing the  soil  surface  level,  and  of  uniform 
height  above  the  steam  outlets,  so  that 
steam  will  not  reach  the  surface  at  one 
point  and  escape,  decreasing  penetration 
at  other  points;  (4)  having  soil  well 
worked  and  of  uniform  moisture  and 
compaction.  In  any  case,  efficiency  is  in- 
creased by  reducing  the  steam  floiv  to  a 
low  level  when  212°  F  is  reached  and  it 
begins  to  escape,  the  so-called  "trickle 
finish." 

Spacing  of  steam  outlets 

The  temperature  rise  to  212°  F  at  a 
point  removed  from  the  steam  input  is 


[155] 


never  instantaneous,  though  in  a  prac- 
tical sense  it  may  appear  to  be.  Even  in 
a  case  reported  by  Morris  and  Winspear 
(1957)  in  which  the  temperature  rose 
from  38.7°  to  212°  F  in  0.5  minute,  the 
reading  at  0.25  minute  was  135.5°.  The 
time  increases  with  increasing  distance 
or  decreasing  steam  flow.  In  one  of  our 
tests  it  required  1.0  minute  to  heat  U.  C. 
mix  C  (50  per  cent  peat)  to  212°  at  2 
inches  from  a  source  injecting  6.7 
pounds  of  steam  per  hour,  and  7.5  min- 
utes at  6  inches;  at  3.5  pounds  of  steam 
1.5  minutes  is  required  at  2  inches,  and 
43  minutes  at  6  inches.  Thus,  the  proper 
spacing  of  perforated  pipes  in  the  soil  is 
determined  in  part  by  the  available 
steam  flow.  Up  to  a  point,  greater  dis- 
tance between  steam  inputs  is  possible 
without  lessened  efficiency,  if  the  steam 
flow  rate  is  increased.  With  a  flow  of 
about  7  pounds  per  hour  from  each  ori- 
fice, the  spacing  might  well  be  12  inches 
or  a  little  more  without  loss  of  efficiency. 
However,  if  the  flow  is  as  low  as  2 
pounds  per  hour,  the  spacing  should  not 
exceed  6  inches  for  a  comparable  effi- 
ciency and  time  (fig.  76). 

The  soil  mass  settles  during  steaming, 
presumably  from  the  increased  weight 
of  the  water  and  from  expulsion  of  air. 
The  settling  may  be  as  much  as  4  inches 
in  20  inches  of  soil,  and  it  commonly  is 
1  inch  or  slightly  more.  This  compaction 
affects  steam  distribution.  In  a  steam-box 
soil  treater  with  a  rigid  pipe  grid  having 
holes  on  the  underside  of  the  pipes  (for 
example,  type  4,  Sec.  10),  this  settling 
of  the  soil  leaves  an  open  space  along 
the  underside  of  the  pipe  after  steam  has 
been  applied  for  a  time.  Steam  fills  this 
space  and  thus  diffuses  into  the  soil  from 
a  line,  rather  than  a  series  of  points.  The 
outward  flow  of  steam  probably  begins 
from  the  several  orifices  and  gradually 
extends  to  become  a  linear  source  along 
each  pipe.  This  would  minimize  the  im- 
portance  of  exact  spacing  of  the  holes  in 
the  pipes  in  such  equipment.  Phis  situa- 
tion  is  not  likely   to  occur  with  buried 


perforated    pipes   or   tiles   because   they 
would  settle  with  the  soil. 

Characteristics   and 
Forms   of  Steam 

Water  is  an  extremely  efficient  medium 
for  the  transfer  of  heat.  It  changes  form 
from  ice  to  water  at  32°  F,  and  above 
that  point  stores  heat  at  the  rate  of  1 
B.t.u.  per  pound  per  1  degree  F  rise,  up 
to  the  boiling  point  (212°).  Thus,  boil- 
ing water  contains  180  B.t.u.  available 
for  soil  heating  above  the  freezing  point 
(fig.  77).  At  212°  another  change  of 
form  occurs,  from  water  to  steam,  and 
for  this  970  B.t.u.  per  pound  are  neces- 
sary, with  no  temperature  increase  (fig. 
77).  As  is  well  known,  water  may  be 
brought  to  the  boiling  point  much  more 
quickly  than  it  can  be  boiled  away,  due 
to  these  heat  requirements.  Thus  steam 
transfers  its  heat  (970  B.t.u)  in  addition 
to  that  of  the  condensed  water  (180 
B.t.u.),  or  about  6.4  times  as  much  as 
does  boiling  water.  About  6.4  times  as 
many  pounds  of  water  must  be  used 
as  steam  to  bring  soil  temperature  from 
32°  to  212°.  This  explains  the  principal 
disadvantage  of  the  hot-water  treatment 
of  soil  (see  "Hot-Water  Drench  of 
Propagating  Sand,"  Sec.  10).  The  prin- 
cipal heat  transfer  by  water  occurs  when 
it  changes  to  steam,  and  vice  versa. 

The  existence  of  an  advancing  front  of 
steam  must  be  appreciated  in  taking  tem- 
perature readings  during  soil  steaming. 
(See  "Movement  of  Steam  through 
Soil,"  above.)  The  temperature  rises 
rapidly  at  a  given  point  under  efficient 
operating  conditions,  and  cold  spots  are 
likely  to  be  untreated  and  at  the  original 
temperature.  Readings  should  be  taken 
at  points  of  slowest  heating,  and  the  tim- 
ing started  when  these  have  reached 
180°  to  212°  F.  If  the  soil  is  adequately 
protected  from  heat  loss,  it  may  not  be 
necessary  to  keep  the  steam  on  after  this 
time.  Thermometers  and  Tempil  Pellets 
for  measuring  temperature  are  described 
in  Section  8  and  the  Appendix. 


156] 


1300 


1200- 


1100- 


1000- 


o 

"o  900 
o 


1107 


800- 


0) 

CO 

C 

<D 

-D 

C 

o 
^   700 


^  600 


■g   500 


400- 


300- 


200- 


100- 


Free-flowing  superheated  steam 


Free-flowing 
dry  saturated 
steam 


_a 

'5 
> 

o 


CO 

o 

a) 

i_ 

D 
to 

CO 

d) 


Steam  and  water 


180 


_o 

_g 

'5 

> 
a 


CO 
00 

o 

ai" 

i— 

D 

CO 
CO 

0) 

k_ 

D_ 

_Q 


J) 
_Q 

'5 

> 

D 


CO 

O 
CN 
O 

© 

CO 
CO 


o 


_Q 

_g 

'a 

> 


o 

o 
o 


l_ 

CO 
CO 

a> 

D_ 

o 

00 


1086 


x 


x* 


Hot  water 


y 


- 


/ 


0 


Ice 


32 


100 


300 


_o 

'5 
> 

o 


CO 

CN 
O 

<0 

i_ 

CO 

CO 

<D 


CO 
CO 
CN 


<1) 

a 

_Q 

_Q 

a 

o 

^~ 

a 

o 

> 

> 

o 

o 

CO 

»o 

CN 
O 


I— 
D 
to 

CO 


O 
CO 


s 


/ 


CO 

CN 
CN 
O 


O 
O 
^O 


/ 


/ 


/ 


.X" 


400 


500 


212 

TEMPERATURE   (°F) 
Fig.  77.  Relative   amount   of   heat   (B.t.u.)    released    by   hot  water,   free-flowing    steam,   steam 
superheated  to  four  different  temperatures,  and  saturated  steam  at  six  pressures.  The  available 
B.t.u.  for  soil  heating  are  indicated  in  each  case.  (Based  on  data  of  Keenan  and  Keyes,   1936; 
Morris,  1954  b,  has  a  similar  graph.) 


[157] 


Free-flowing   or 
pressureless  steam 

The  conversion  from  water  to  steam  is 
accompanied  by  a  1600-fold  increase  in 
volume.  If  this  change  occurs  in  a  boiler, 
the  degree  of  pressure  may  be  regulated 
by  controlling  the  rate  of  fuel  supply  or 
of  steam  flow. 

Free-flowing  steam  without  pressure 
or  superheating  may  deliver  1,150.4 
B.t.u.  per  pound  to  an  object  at  32°  F 
on  which  it  condenses.  Of  this  amount 
180.07  B.t.u.  represents  the  heat  residual 
in  water  at  212°,  and  970.3  B.t.u.  the 
heat  released  when  steam  condenses  to 
water  (fig.  77).  When  steam  is  injected 
into  soil  at  212°  the  heat  is  not  trans- 
ferred from  the  water  since  the  soil  itself 
is  raised  to  212°,  heat  flowing  only  from 
a  warm  to  a  cooler  object.  Only  970 
B.t.u.  per  pound  are  therefore  available; 
but  when  soil  is  heated  to  only  180°, 
there  are  1,002  B.t.u.  available. 

Steam  delivered  from  the  boiler 
through  pipes  is  commonly  under  pres- 
sure in  order  to  deliver  it  in  adequate 
amounts.  At  15  pounds'  boiler  pressure 
the  temperature  of  steam  is  249.8°  F. 
However,  when  such  steam  is  released 
into  the  soil,  the  pressure  is  immediately 
lost  and  the  temperature  drops  back  at 
once  to  about  240°  and  then  to  212°. 
As  this  steam  condenses  it  will  yield  only 
about  14  B.t.u.  more  per  pound  of  dry 
steam  than  at  212°  (fig.  77).  The  extra 
heat  content  from  the  pressure  is  briefly 
converted  to  superheat,  and  tends  to  dry 
the  steam  by  evaporating  the  water  drops 
it  contains.  The  end  result  is  usually, 
therefore,  to  supply  slightly  more  and 
drier  steam  to  the  soil.  Its  effect  on  the 
organisms  is  the  same  as  the  free-flowing 
type,  since  they  are  subject  to  no  pres- 
sure. 

Steam  under  pressure 

I  he  flow  may  be  restrained  so  that 
boiler  pressure  will  be  built  up,  with 
some  increase  in  the  heat  available  for 
soil  steaming.  Thus,  at  80  pounds  per 


square   inch   boiler   pressure,   there   are 
1,006  B.t.u.  available  (fig.  77). 

Steam  under  pressure  is  most  com- 
monly used  in  cannery  retorts  or  auto- 
claves operating  at  15  pounds'  pressure. 
Although  this  is  an  effective  type  of 
equipment,  the  gain  in  heat  transfer  does 
not  justify  the  cost  of  a  steamtight  sys- 
tem. Only  14  B.t.u.  more  per  pound  of 
dry  steam  are  delivered  in  such  auto- 
claves than  by  free-flowing  steam  (fig. 
77) .  If  the  pressure  is  increased  to  more 
than  15  pounds,  the  equipment  becomes 
excessively  expensive  and  there  may  be 
restrictions  to  its  operation.  Further- 
more, there  is  no  effective  decrease  in 
time  of  treatment  or  gain  in  efficiency, 
because  the  autoclave  must  be  operated 
with  the  exhaust  valve  or  door  open  for 
a  time  to  free  it  of  air  pockets  before 
pressure  is  built  up  (see  "Relation  of 
steam/air  ratio  to  condensation,"  above) . 

Superheated  steam 

Steam  may  be  superheated  by  passing 
it  through  the  fire  box  to  heat  it,  much 
as  a  furnace  heats  air.  Because  the 
specific  heat  of  steam  is  about  half  that 
of  water,  there  is  a  gain  of  only  about 
47  B.t.u.  for  each  100  degrees  of  super- 
heat (fig.  77).  At  300°  F  the  gain  would 
be  only  about  43  B.t.u.  This  4.4  per  cent 
gain  causes  almost  no  noticeable  de- 
crease in  time  of  treatment,  but  con- 
siderably increases  the  cost  of  equipment. 
Steam  superheated  to  about  450°  was 
used  for  a  time  in  one  commercial  soil- 
treatment  operation  in  southern  Califor- 
nia; this  gave  an  increase  of  about  114 
B.t.u.  (about  11.8  per  cent)  available  for 
soil  treatment  per  pound  of  steam  over 
the  free-flowing  type. 

In  general,  however,  superheating  is 
more  effective  than  high  pressure  in  in- 
creasing the  heat  content  of  steam;  at 
400°  F  it  is  3.8  per  cent,  and  at  500° 
8.3  per  cent  better.  Superheated  steam 
contains  no  unvaporized  water  and, 
therefore,  does  not  make  the  soil  quite 
as    wet   as   does   steam    under   pressure. 


I  L58  I 


This,   however,   is   not   a   critical   factor 
with  good  nursery  soils. 

It  is  questionable  whether  there  is 
enough  gain  over  free-flowing  steam  at 
212°  F  from  either  steam  under  pressure 
or  superheated  to  justify  the  increased 
cost;  in  the  ranges  shown  in  figure  77 
the  gain  is  only  1.4  to  14.1  per  cent. 

Volume  of  Steam  Required 

It  is  desirable  to  have  a  boiler  and 
steam  pipes  of  sufficient  capacity  that 
quantity  of  steam  will  not  be  seriously 
limiting  at  any  time  in  soil  treatment. 
This  means  that  the  higher  the  boiler 
horsepower  rating  (steam-producing 
capacity)  the  faster  a  given  soil  mass 
can  be  heated,  or  the  larger  the  mass 
that  can  be  heated  in  a  given  time.  One 
boiler  horsepower  is  the  capacity  to  con- 
vert 34.5  pounds  of  water  at  212°  F  per 
hour  into  steam  at  0-pound  gauge;  it 
equals  33,475  B.t.u.  per  hour.  It  is  now 
customary  to  rate  boilers  in  pounds  of 
steam  generated  per  hour.  Boiler  ca- 
pacity bears  no  relation  to  steam  pres- 
sure, and  a  satisfactory  boiler  may  be 


of  the  flash  type,  without  pressure,  pro- 
vided the  steam  does  not  have  to  travel 
a  long  distance  through  pipes.  Since 
there  is  little  gain  in  heat  transfer 
from  using  steam  under  pressure  or  in 
superheated  condition  (fig.  77),  the  best 
way  to  obtain  the  necessary  soil-heating 
capacity  is  to  use  a  boiler  of  adequate 
size. 

Steaming  too  much  soil  for  the  boiler 
capacity  is  inefficient  owing  to  heat 
losses  through  radiation,  transmission, 
and  convection,  as  is  permitting  exces- 
sive steam  loss  because  of  too  small  a 
load.  A  balance  must  be  worked  out  for 
each  piece  of  equipment,  between  too 
much  and  too  little  steam  for  the  volume 
of  soil  treated.  The  assistance  of  a  heat- 
ing engineer  is  helpful  in  calculating  the 
required  boiler  capacity  for  a  given  soil- 
steaming  operation.  However,  a  grower 
can,  to  a  large  extent,  adjust  to  the 
capacity  of  a  given  boiler  by: 

1.  Decreasing  the  area  (in  benches)  or 
volume  (in  bulk  steaming  equipment)  of 
soil  treated,  if  it  requires  more  than  1 
hour  to  raise  the  temperature  to  212°  F. 


« 

Table  14.  The  Time  Required  to  Bring  a  U.  C.-Type  Soil  Mix  to  212°  F, 
and  the  Amount  of  Soil  That  Can  Be  So  Heated  in  1  Hour 

For  7  different  boiler  capacities  and  3  levels  of  efficiency  in  heat  exchange* 

Boiler  capacity 

Equivalent 
kilowatt- 
hours  t 

Time  per  cu.  yd.  to 

raise  temperature  to 

212°  F  at  3  efficiency 

levels;  in  min. 

Maximum  soil  heated  in  1  hr. 
at  3  efficiency  levels;  in  cu.  yd. 

Lb.  steam 
per  hr. 

Calculated 
boiler 
horse- 
power t 

30% 

50% 

70% 

30% 

50% 

70% 

100 

2.9 

28.4 

175 

105 

75 

0.34 

0.57 

0.80 

200 

5.8 

56.9 

88 

53 

38 

0.68 

1.14 

1.60 

300 

8.7 

85.3 

58 

35 

25 

1.03 

1.71 

2.39 

500 

14.5 

142.2 

35 

21 

15 

1.71 

2.85 

3.99 

1,000 

29.0 

284.4 

18 

11 

8 

3.42 

5.70 

7.98 

2,500 

72.4 

710.9 

7 

4 

3 

8.55 

14.26 

19.96 

5,000 

144.9 

1,421.9 

4 

2 

2 

17.11 

28.51 

39.92 

*  Computed  on  basis  of  15  per  cent  water  content  in  soil,  150  degree  F  rise  in  temperature,  and  specific 
heat  of  0.2. 

t  Computed  on  basis  of  33,475  B.t.u.,  or  34.5  lb.  steam,  per  boiler  horsepower  at  100  per  cent  efficiency. 
Because  boilers  are  often  rated  on  the  basis  of  area  of  heating  surface,  without  regard  to  efficiency,  these 
figures  may  bear  little  relation  to  commercial  horsepower  ratings. 

t  Calculated  on  basis  of  3,411  B.t.u.  per  kilowatt-hour  at  100  per  cent  efficiency. 

[159] 


2.  Increasing  the  area  or  volume  of 
soil,  increasing  the  number  of  steam  out- 
lets in  the  mass,  or  simply  reducing  the 
steam  flow  with  a  valve,  or  the  pressure 
with  a  regulator,  if  steam  is  escaping  in- 
stead of  condensing. 

The  efficiency  of  the  operation  de- 
pends on  the  proper  balancing  of  all  of 
the  factors  by  the  grower. 

Table  14  gives  data  on  the  time  re- 
quired for  soil  steaming  with  boilers  of 
various  sizes,  and  for  heat-exchange  sys- 
tems of  different  levels  of  efficiency.  It 
also  presents  data  on  the  amount  of  soil 
that  can  be  treated  in  each  case.  Cal- 
culated for  a  soil  mix  of  the  U.  C.  type, 
this  gives  a  steam  requirement  of  10.8 
pounds  per  cubic  foot  of  soil  at  30  per 
cent  efficiency,  6.5  pounds  at  50  per  cent, 
and  4.6  pounds  at  70  per  cent  efficiency 
to  heat  150  degrees  F.  Bunt  (1954-55) 
found  the  requirement  to  be  5.40  to  8.45 
pounds  per  cubic  foot  (average  6.51)  to 
heat  clay  loam  158  degrees. 

A  soil  mix  of  the  U.  C.  type  with  15 
per  cent  moisture  would  require  70  B.t.u. 
per  cubic  foot  per  degree  rise  in  tem- 
perature at  30  per  cent  efficiency,  42 
B.t.u.  at  50  per  cent,  30  B.t.u.  at  70  per 
cent,  and  21  B.t.u.  at  100  per  cent  effi- 
ciency. Morris  (1954a)  obtained  figures 
ranging  from  24  B.t.u.  per  cubic  foot 
per  degree  for  compact,  dry,  light  soil 
at  9  per  cent  moisture,  up  to  53  B.t.u. 
for  compacted  heavy  soil  at  58  per  cent 
moisture. 

A  safe  working  figure  for  steam  re- 
quirement in  heating  soil  would  appear 
to  be  6.5  pounds  per  cubic  foot,  or  42 
B.t.u.  per  cubic  foot  per  degree  F. 

Proper  Soil  Condition  for 
Steaming 

Moisture  content 

The  moisture  content  of  soil  to  be  heat 
treated  is  of  great  importance  in  three 
different  ways. 

I.  //  requires  about  five  times  as  many 
B.t.u.  to  heat  J  pound  of  water  as  it 


does  1  pound  of  soil.  The  specific 
heat  of  a  light  sandy  soil  has  been 
reported  by  Morris  (1954a)  as 
0.192  and  a  heavy  clay  soil  as  0.202 
(both  oven-dried),  in  comparison 
with  approximately  1.0  for  water. 
A  peat  soil  had  about  the  same 
value  as  the  heavy  clay  soil.  An 
average  of  0.2  is  generally  used  for 
soils.  Thus,  to  raise  1  pound  of  an 
average  soil  1  degree  F  requires 
about  0.2  B.t.u.  A  soil  with  20  per 
cent  moisture  requires  about  as 
many  B.t.u.  to  heat  the  water  as  it 
does  the  soil,  despite  the  fact  that 
the  water  accounts  for  only  about 
one  sixth  of  the  total  weight.  Ex- 
cessively wet  soil  often  requires 
twice  as  long  to  reach  212°  F  as  one 
in  good  planting  condition,  and  it 
is  therefore  uneconomic  to  steam 
soil  while  it  is  soggy. 

2.  Heat  plus  moisture  is  much  more 
effective  in  killing  microorganisms 
than  is  heat  alone,  and  soil  should, 
therefore,  not  be  excessively  dry 
when  treated. 

3.  Heat  conduction  of  soil  improves 
with  increasing  moisture  content, 
and  treatment  therefore  becomes 
slightly  more  efficient. 

As  a  practical  compromise  between 
these  opposing  conditions,  the  soil  to  be 
steamed  should  have  sufficient  moisture 
to  be  in  good  planting  condition,  that  is, 
after  being  squeezed  in  the  hand  it  will 
crumble  easily.  When  such  soil  is 
steamed  it  comes  out  in  good  condition 
for  planting,  without  wasteful  heating  of 
excess  water,  and  with  satisfactory  de- 
struction of  microorganisms. 

Soil  structure 

The  structure  of  soil  is  important  in 
heat  treatment  because  it  affects  the  pas- 
sage of  steam  and  the  conduction  of  heat 
(see  "Manner  of  Heat  Distribution," 
and  "Movement  of  Steam  through 
Soil,"  above) . 

A    clod    is    compressed,    has    reduced 


L60  | 


pore  size,  and  therefore  presents  special 
problems.  It  may  be  surrounded  as  the 
steam  margin  advances,  since  it  reaches 
212°  F  throughout  its  mass  more  slowly 
than  does  the  porous  soil.  The  time  re- 
quired increases  with  the  size  and  com- 
pactness of  the  lump,  since  steam  diffuses 
inwardly,  and  air  is  expelled  simulta- 
neously through  the  reduced  pores. 
Morris  (19546)  found  that  a  lump  3% 
inches  in  diameter  did  not  reach  160° 
at  the  center  while  the  surrounding  soil 
reached  212°;  one  5%  inches  in  diam- 
eter took  50  minutes  to  reach  212°  at  the 
center;  one  7  inches  in  diameter  took 
more  than  1  hour  to  reach  160°  at  the 
center.  If  lumps  are  covered  by  at  least 
2  inches  of  porous  soil,  they  may  be 
heated  through  from  transferred  heat 
during  the  after-cooking.  They  should, 
however,  be  broken  up  as  much  as  pos- 
sible or  removed  by  screening,  to  reduce 
the  chances  of  imperfect  heating  and  of 
"blow-through"  by  steam.  Hoare  (1953) 
considered  that  the  heating  of  clods, 
even  by  this  slow  diffusion  of  steam,  was 
rapid  as  compared  with  the  rate  of  heat 
transfer  by  conduction. 


Because  uneven  packing  of  the  soil  in 
the  container  also  makes  for  uneven 
heating,  particularly  at  high  rates  of 
steam  flow,  it  should  be  avoided  (see 
"Movement  of  Steam  through  Soil," 
above). 

Soil  must  be  as  free  of  clods  as  pos- 
sible for  fast,  successful,  and  economical 
steaming.  A  U.  C.-type  mix  uses  soil  of 
such  texture  that  resistant  clods  are  not 
formed  (Sec.  6),  and  the  whole  problem 
is  thus  avoided.  If  lumpy  soils  must  be 
used,  they  should  be  pulverized  or 
screened  before  being  steamed. 

In  heating  dry  soil  the  picture  is  quite 
different;  heat  is  transmitted  from  par- 
ticle to  particle,  and  the  smaller  the  pore 
space  the  better.  However,  this  treatment 
method  is  inefficient  for  other  reasons, 
and  is  now  little  used.  In  treatment  of 
moist  soil  with  heat,  part  of  the  thermal 
transfer  is  by  steam,  as  explained  under 
"Treatment  of  Soil  by  Heat — Rate  of 
Heat  Distribution,"  above.  The  greater 
moisture  capacity  of  clay  than  sandy  soils 
is  more  important  in  determining  the 
greater  number  of  B.t.u.  required  to  heat 
them  than  is  the  porosity  or  weight  per 
cubic  foot. 


[161] 


SECTION 


Equipment  for  Heat 
Treatment  of  Soil 


Kenneth  F.  Baker 
Chester  N.  Roistacher 

Considerations  in  the  choice  of  equipment 
Stationary  soil  mass  treated  in  batches 
Stationary  soil  mass  in  benches  or  beds 
Moving  soil  mass  in  continuous  output 
Moving  soil  mass  treated  in  batches 
Equipment  for  generating  and  distributing  steam 
Soil  treatment  in  a  mechanized  nursery 


ROM  THE  discussions  in  the  two  pre- 
ceding sections  it  is  evident  that  there 
can  be  no  one  type  of  soil  heat-treating 
equipment  that  is  best  for  all  purposes. 
There  are  almost  as  many  different 
special  models  of  equipment  as  there  are 
growers  treating  soil. 

The  grower  must  decide  whether  to 
use  a  continuous  type  with  a  moving  soil 
mass  or  a  batch  type  with  static  soil 
mass,  whether  soil  is  to  be  treated  in 
bulk  or  in  the  containers,  whether  the 
unit  is  to  be  stationary  or  mobile, 
whether  steam  or  dry  heaters  are  to  be 


used,  and  whether  the  fuel  is  to  be 
natural  gas,  butane,  oil,  propane,  or 
electricity.  It  is  a  matter  of  finding  the 
type  best  suited  to  the  given  operation. 
To  provide  the  facts  on  which  such  a 
choice  must  rest,  the  principles  have 
been  discussed,  and  specific  equipment 
involving  them  is  now  presented. 
Growers  often  develop  equipment  them- 
selves, and  rediscover  designs  already 
abandoned  by  others.  All  basic  types 
are  therefore  described,  even  though 
some  are  not  recommended.  References 
are  given  so  that  further  details  may  be 
obtained. 


CONSIDERATIONS  IN  THE  CHOICE  OF  EQUIPMENT 


Stationary   versus 
moving    soil    mass 

All  of  the  types  of  equipment  used  by 
growers  may  be  grouped  into  two 
classes,  according  to  whether  the  soil 
mass    is   stationary    or   moving.   On   the 


basis  of  efficiency,  dependability,  low 
cost,  and  minimum  recontamination 
hazard  we  consider  that  the  best  type  is 
that  in  which  the  soil  is  treated  by  flow- 
ing steam  in  planting  containers  (flats, 
pots,  beds)  or  /'//  stationary  piles  (steam 


[162] 


chambers,  autoclaves,  Thomas  method). 
It  does  not  follow,  however,  that  all 
nurseries  should  use  one  of  these  types. 

//  the  heat  source  is  steam,  types  of 
equipment  with  a  stationary  soil  mass 
will  prove  best  in  most  cases.  If  the 
source  is  a  dry  heater  of  some  sort, 
thermal  transmission  will  be  much  more 
efficient  and  better  controlled,  with  either 
moist  or  dry  soil,  if  equipment  using  a 
moving  mass  of  soil  is  employed.  Conse- 
quently, equipment  of  this  type  is  con- 
sidered best  when  dry  heaters  and  either 
dry  or  moist  soil  are  used. 

As  explained  in  Section  9,  it  is 
presently  impractical  with  stationary- 
type  equipment  involving  the  use  of 
steam  to  heat  a  soil  mass  uniformly  to 
less  than  212°  F,  although  this  can  be  ac- 
complished with  equipment  having  a 
moving  soil  mass.  If  heat  is  applied  to 
dry  soil,  the  temperatures  may  rise  well 
above  this  point.  With  equipment  treat- 
ing a  moving  soil  mass  it  is  possible  to 
terminate  the  process  at  any  desired 
temperature  by  varying  the  heat  input 
(through  control  of  the  steam,  gas,  oil, 
or  electricity)  or  the  time  of  exposure 
(through  regulation  of  the  speed  with 
which  soil  is  moved). 

Provision  for  after-cooking 

Merely  heating  soil  to  180°  to  212°  F 
is  not  sufficient;  it  must  be  kept  at  that 
temperature  for  30  minutes.  Equipment 
for  continuously  treating  a  moving  soil 
mass  rapidly  heats  a  small  quantity  of 
soil  and  then  dumps  it.  Provision  must 
always  be  made  to  keep  the  soil  tempera- 
ture at  180°  to  212°  F  for  at  least  30 
minutes.  This  can  be  accomplished  by 
quickly  stacking  the  filled  flats  and 
covering  with  a  clean  heavy  canvas,  or 
by  similarly  covering  the  pile  of  soil. 
Batch  equipment  for  treating  a  sta- 
tionary soil  mass  usually  provides  for 
this  after-cooking,  but  separate  arrange- 
ments must  be  made  for  the  continuous 
output  from  moving-soil  equipment. 


Is  a  boiler  already  available? 

In  general,  there  are  two  types  of 
California  nurseries  with  reference  to  the 
use  of  steam.  One  group  operates  glass- 
houses and  has  a  large  steam  boiler  for 
heating  the  range,  or  plans  to  install  one. 
The  other  and  more  common  type  does 
not  have  steam-generating  equipment, 
either  because  only  lath  houses  and  out- 
door plantings  are  used,  or  because  the 
glasshouses  are  heated  by  vented  gas 
stoves  or  unit  heaters.  The  first  group 
may  use  their  existing  facilities,  but  the 
latter  must  either  procure  a  boiler  or  use 
the  self-generating  or  dry-heater  types  of 
equipment  discussed  in  this  section. 

Permanent  versus 
mobile  equipment 

A  question  that  frequently  arises  is 
whether  to  place  the  soil  treatment  and 
handling  equipment  in  a  permanent  loca- 
tion in  a  nursery,  or  to  maintain  a 
mobile  unit.  There  are  many  modifica- 
tions of  each  equipment  type  in  use,  and 
the  decision  must  rest  with  the  grower. 
Experience  has  indicated,  however,  that 
there  is  less  trouble  with  the  boiler, 
mechanical  parts,  and  recontamination 
when  the  soil  is  taken  to  the  equipment, 
rather  than  vice  versa.  With  present 
conveyer  systems  an  efficient  mechanized 
nursery  can  be  built  around  a  single 
soil-treating  installation    (Sec.  17). 

Separating  various  operations 

It  is  better  if  the  various  operations  in 
soil  preparation  are  physically  separated 
in  some  way,  so  as  to  minimize  recon- 
tamination. For  example,  several  nur- 
series have  divided  them  as  follows: 

1.  A  soil  yard  where  storage  and  mix- 
ing take  place.  The  flats  or  cans  may  be 
filled  there  if  treating  is  to  be  done  in 
the  container,  and  a  mobile  bulk  cooker 
may  also  be  filled  there. 

2.  The  treating  should  be  done  at 
another  location  or  preferably  in  a 
building  so  as  to  minimize  wind-blown 


[163] 


Table  15.  Summary  of  Characteristics  of  Equipment 

+  =  yes;  -  =  no;  —  +  =  both  yes  and  no  apply  because  of  dual  function; 

Characteristics 

Equipment  types*  with  soil  stationary,  handled  in  ' 

latches 

External  steam  source 

Generates  own  steam 

3 
Pi 

1 

a 
A 
© 

A 
O 

S 

2* 

A 
o 

a 
B 

a 

>x 
a 

'£ 

o 

p. 

1 

c 

m 
3 

M 

•3 

A 

M 

O 

Jo 

a 
§ 

w  n 

4a* 

3 
A 

K  2 

0  a 

■°-s 

8a 
a  o 

01  o 

4b* 

M 

£m 

.  hi 
M    J) 

*  s 
11 
5* 

3 

■8 
> 

6* 

4) 
N 

O 

o. 
o, 

Sis 
7* 

4) 

a 

A 
d 

B 

"3 
u 

B 

4) 

> 

8 

0) 

> 

73 
o 

3 
< 

9 

45  t) 
S  O 

■ -o 

It 

°A 

2  <« 

c  > 
o  o 
XB 

10 

e 

c 

A 
el 
@ 

*3 
s 

E 

4) 

> 

11 

C 

6 
12 

1 

a 
o 
■ 

o 
X 

13 

Soil  treated  in  containers 

-  + 

-  + 

- 

- 

-  + 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

-  + 

Treats  containers  separately 

+ 

+ 

+ 

- 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

Powered  moving  parts  used 

Steam  efficiently  used 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+? 

+ 

+ 

+ 

Equipment  inexpensive 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

- 

+ 

- 

+ 

+ 

+ 

+ 

Steam  put  into  or  surrounding  soilf 

IS 

IS 

I 

I 

IS 

S 

S 

S 

S 

S 

s 

s 

S 

S 

Soil  heated  above  212°  Ft 

Easy  to  load  and  unload 

- 

+ 

+ 

+ 

+  - 

+ 

+ 

- 

- 

- 

+ 

- 

- 

- 

Type  of  steam  used§ .  . 

FPS 

FPS 

FPS 

FPS 

FPS 

FPS 

FPS 

FPS 

FPS 

P 

F 

F 

F 

F 

Minimized  recontamination  hazard 

-  + 

-  + 

- 

- 

-  + 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

-  + 

Portable  unit^ 

- 

+ 

+ 

-  + 

+ 

+ 

- 

- 

- 

- 

- 

- 

- 

-  + 

Also  used  for  hot-water  treatment  of  stock 

- 

- 

- 

- 

- 

- 

- 

+ 

- 

- 

- 

- 

- 

- 

Fits  into  mechanization 

- 

+ 

+ 

+ 

+  - 

+ 

+ 

- 

- 

+ 

+ 

- 

- 

- 

Type  of  fuel  or  electricity  used|| 

GP 

GE 

GP 

GPCE 

Danger  of  burns  or  shocks 

- 

- 

- 

- 

- 

- 

- 

- 

- 

+ 

- 

- 

- 

- 

Treat  around  posts  easily 

Effective  deep  treatment  of  soil 

Useful  in  small  or  large  operations**. . . . 

SL 

SL 

SL 

SL 

S 

SL 

SL 

S 

S 

L 

S 

S 

S 

S 

L64  | 


for  Heat  Treatment  of  Soil.  See  Text  for  Details 

?  =  variable  or  uncertain;  blank  =  does  not  apply. 


Equipment  type 

* 

Soil  in  benches  or  beds 

Soil  moving 

Continuous  type 

Dry  source  of  heat 

Steam 

Hot 
water 

Steam- 
formal- 
dehyde 

Steam 

Dry  source  of  heat 

Batch 
type 

bo 
B 

a 

-B  M 

st 

5  S 

u  E 

«  a 
w» 

14 

■ 

0 

& 

9 
■ 

3 

15 

!2 

M 

a 
_o 

o 

3 

•a 

c 

16 

bo 

e 

S 
a 

» 

17 

■ 

s 

o 

J3 
H 

18* 

a 

03 
O. 

■ 

• 

a 
19* 

B 

N 

O 

B 

a 

Q. 

■ 
3  & 

«a 

20 

s 

M 

a 

M 
M 

0 
■ 

— 
w 

21 

• 

•a 

V 

= 

m 
22 

a 
M 
OS 
N 

bo 

a 

'> 

o 

s 

23 

•a 

a 

M 

a 

t» 

a 

a 

E 

is 

w  ft 
24 

i 

h 

a 

S  a 

flj   03 

«  a 

25 

"3 

0 

I 

fi    hi 

Oi  — 

•"  o 

W  [> 

26 

o 

a 
a 

Cm 

£j 
MS 
27 

Bo 

a  ■ 

%* 
I? 

3  * 

ei.5 
28 

* 

a 
g 

H 

■ 

bo 

a 

3 
o 
PS 

29* 

8  a 

c  "3 
'3  a 
*»  a 

II 

30 

as 

a  E 
£  a 
^c 

be— 

a  <* 
11 

Sa 

K  a 

31 

a 
o 

s 

a 

a 
a 

S 

32 

£ 

a 

hi 

a 

■ 

bo 
Bo 

3o 

O    - 
—     - 

33 

*a 

£0 

-   oS 

mc 

Si 

K  a 

34 

B  a 

=  = 

bo_ 

C     =« 

B  a 

—  a 

9   M 

tf  a 

35 

- 

- 

- 

- 

+ 

+ 

+ 

+ 

+ 

- 

+ 

+ 

+ 

+ 

- 

- 

- 

- 

+ 

+ 

- 

- 

- 

- 

+ 

+ 

+ 

- 

- 

- 

- 

- 

- 

- 

- 

- 

+ 

- 

- 

- 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

- 

+ 

+? 

+? 

+ 

+ 

+ 

- 

— 

+ 

+ 

+ 

+ 

- 

- 

- 

+ 

+? 

S 

S 

I 

I 

I 

I 

S 

I 

S 

I 

I 

I 

+ 

- 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

- 

+ 

+ 

+ 

- 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

+? 

FPS 

FPS 

FPS 

FPS 

FPS 

FPS 

F 

FPS 

FS 

FPS 

FPS 

F 

- 

- 

- 

- 

+ 

+ 

- 

- 

+ 

+ 

+ 

+ 

+ 

+ 

- 

- 

- 

- 

+ 

+ 

+ 

+ 

- 

+ 

- 

+ 

+ 

+ 

+ 

+ 

+ 

+ 

- 

- 

+ 

- 

- 

- 

- 

- 

+? 

+ 

+ 

+ 

+ 

+? 

+? 

+? 

+ 

E 

E 

E 

GCP 

E 

PG 

GP 

P 

GP 

E 

E 

GP 

P 

- 

+ 

- 

- 

- 

+ 

+ 

+ 

- 

- 

+ 

+ 

+ 

- 

- 

- 

+ 

+ 

- 

- 

- 

- 

+ 

+ 

- 

- 

- 

+ 

+? 

+ 

+ 

- 

- 

+ 

S 

S 

S 

S 

SL 

SL 

SL 

SL 

L 

L 

S 

SL 

SL 

S 

S 

S 

S 

S 

S 

S 

S 

S 

*  Numbers  refer  to  equipment  types  in  text;  asterisks  indicate  best  types  for  California  conditions. 
t  I  =  into;  S  =  surrounding. 

X  May,  of  course,  be  heated  above  2123  F  if  superheated  steam  is  applied  for  a  long  enough  period. 
§  F  =  no  pressure,  free-flowing;  P  =  pressure;  S   =  superheated. 

^  Unit  may  be  portable,  but  cannot  be  used  away  from  special  electric  wiring  (for  example,  14,  15,  16).  If  units  are  portable  when  pro- 
vided with  a  mobile  steam  source,  they  are  so  classed  here. 
I!  P  =  type  of  petroleum;  E  =  electricity;  G  =  gas;  C   =  coal. 
**  S  =  small  operations  only;  L  =  adaptable  to  large  operations. 


[165] 


dust.  Filling  of  treated  flats  or  cans  with 
treated  soil  may  be  done  at  the  same 
place  or  in  the  planting  shed. 

3.  Planting  of  the  containers  usually 
is  done  in  another  room. 

With  this  compartmentalization  of  the 
process  there  is  small  chance  of  dust 
from  the  soil  yard  blowing  into  treated 
soil.  Care  should  be  exercised,  however, 
to  see  that  adhering  untreated  soil  is  not 
carried  on  the  mobile  cooker  from  phase 
1  to  3.  There  should  be  a  concrete  or 
wood  floor  for  phases  2  and  3,  and  this 
might  well  be  hosed  down  every  day. 
When  treated  soil  is  dumped  in  bulk 
piles  on  the  floor,  the  surface  should  pre- 
viously have  been  wet  down  with  a 
formaldehyde  solution  (1  gal.  to  18  gal. 
water) . 

Adapting  batch  equipment 
to  continuous  operation 

It  is  possible  to  capitalize  on  the  effi- 
ciency inherent  in  equipment  treating  a 
stationary  mass  of  soil,  and  yet  operate 
on  a  continuous-batch  system.  This  was 
done  by  a  large  commercial  unit  in 
southern  California  that  operated  with 
the  method  of  the  steam  box  (type  4) . 
Two  steam  boxes  were  operated  in  se- 
quence, one  being  filled  with  soil  while 
the  other  was  cooking.  Since  the  soil  was 
dumped  directly  into  flat-filling  equip- 
ment, an  almost  continuous  flow  of 
treated  filled  flats  was  produced. 

Something  of  the  same  sort  might  be 
done  with  the  mobile  bins  (type  2),  one 
mobile  bin  being  taken  after  treatment 
and  dumped  into  the  flat-  or  can-filling 


equipment  while  the  other  was  being 
filled  and  steamed.  Two  vault  units  (type 
6)  could  be  similarly  operated  in  se- 
quence, and  the  work  mechanized  by 
placing  the  containers  on  pallets  han- 
dled by  fork-lift  tractors.  This  would 
keep  the  boiler  and  the  crew  almost  con- 
tinuously active  if  the  equipment  were 
properly  designed.  The  same  could  be 
done  with  two  autoclaves  (type  9) 
operating  in  sequence,  but  the  initial  cost 
would  be  high. 

The  important  point  is  that  it  is  not 
necessary  to  use  equipment  with  a  mov- 
ing soil  mass  in  order  to  achieve  con- 
tinuity of  operation,  with  the  attendant 
benefits  of  mechanization  (Sec.  17). 

Use  of  superheated  steam 

There  is  slightly  greater  heat  trans- 
mission and  decreased  water  content  in 
superheated  than  in  free-flowing  steam 
(Sec.  9).  While  it  is  not  commonly  used 
because  of  the  greater  cost  of  the  boiler, 
it  can  be  used  with  many  of  the  types  of 
equipment  described  here,  without  their 
modification  in  any  way.  Superheated 
steam  may  be  used  with  equipment  types 
1  to  8,  18  to  23,  and  26  to  28  described 
below. 

Summary  of  Equipment 

A  summarized  statement  of  the  char- 
acteristics, advantages,  and  disadvantages 
of  the  35  types  of  equipment  is  given  in 
table  15.  One  should  use  this  tabulation 
to  determine  the  types  of  equipment  with 
the  necessary  features  for  a  given  instal- 
lation, then  refer  to  the  text  for  details. 


STATIONARY  SOIL  MASS  TREATED  IN  BATCHES 


Other  things  being  equal,  the  sta- 
tionary types  are  cheaper,  easier  to  main- 
lain,  do  not  require  power  to  drive 
moving  parts,  and  are  less  likely  to  break 
down  at  crucial  times,  than  are  those 
will)  a  moving  ^<>il  mass.  In  general,  they 


are  also  more  efficient  in  the  use  of  steam 
because  it  is  less  likely  to  escape  from 
inside  a  static  soil  mass  (before  it  has 
reached  212°  F)  than  from  a  moving 
mass  intermittently  exposed  to  air.  Types 
I  I  through  17  have  a  dry  source  of  heat. 


[166] 


Pressureless  Steam  from  External 
Source  Released  into  Soil 

1.  The  Rudd  type  .  .  . 

first  used  in  Illinois  in  1893,  repre- 
sents the  simplest  form  of  this  equip- 
ment. It  has  been  redescribed  by  several 
stations  in  England,  Europe,  and  the 
United  States,  often  as  a  new  develop- 
ment. It  consists  essentially  of  a  bin  con- 
structed of  wood,  cement,  or  brick,  lo- 
cated on  the  ground  so  that  it  can  be 
loaded  and  unloaded  from  either  the  top 
or  one  side  (fig.  78).  It  is  preferably 
covered  with  a  hinged  lid.  The  dimen- 
sions vary  considerably,  but  the  soil 
depth  should  not  greatly  exceed  12  in. 
Pipes  of  1-in.  diameter  are  laid  in  open 
channels  in  the  floor  about  9  in.  apart, 
and  are  drilled  on  the  underside  with 
%-in.  holes  9  in.  apart.  The  channels 
may  be  covered  by  boards  with  V2_m- 
holes.  Alternatively,  the  pipes  may  be 
partly  imbedded  in  the  concrete  floor, 
with  the  holes  at  the  top;  the  pipes  must 
then  lead  into  a  condensate  header  from 
which  the  soil  may  be  blown  before  use. 
The  floor  should  slope  slightly  toward 


one  corner  for  drainage  of  condensate. 
Such  a  unit  may  be  used  with  free-flow- 
ing, pressure,  or  superheated  steam, 
operating  in  all  cases  without  pressure 
in  the  box.  If  used  for  soil  in  containers, 
this  becomes  the  vault  type  (type  6).  In 
Norway  a  box  with  a  steam  grid  in  the 
bottom  is  pivoted  at  the  ends  so  that  the 
box  may  be  easily  tipped  and  dumped 
after  steaming.  This  idea  might  be  ex- 
tended to  the  steam  box  (type  4a)  with 
benefit.  Advantages:  very  efficient  use 
of  steam,  simple  and  inexpensive;  de- 
pendable; no  moving  parts;  can  be  used 
for  steaming  pots  and  flats;  may  be 
loaded  by  machinery.  Disadvantages: 
soil  must  be  shoveled  out,  greatly  in- 
creasing recontamination  hazard  and 
labor  cost.  Does  not  fit  well  into  a  fully 
mechanized  schedule.  Best  use:  treating 
soil  in  containers;  treating  containers. 
References:  Rudd  1 1893)  ;x  Fosler 
(1950);  Bewley  (1948,  p.  13-15,  fig. 
11);  Lawrence  and  Bunt   (1955). 

A  modification  of  this  is  to  use  tiles 
instead  of  pipe,  and  use  a  deeper  mass 

1  See  Appendix  for  complete  references,  cited 
here  by  author  and  date. 


Fig.  78.  The   Rudd  type  of  steamer  for  stationary   bulk  soil   (type    1).   Fig.  79.  The  fixed-front 
steam  box  for  stationary  bulk  soil  (type  4a).  See  also  fig.  81. 


[167] 


of  soil.  References:  Ball  (1942,  p.  3)  ; 
Roll-Hansen  (1949,  p.  7-8). 

*2.  The  mobile-bin  type... 

used  by  several  California  growers  elimi- 
nates the  above  disadvantages.  The  bin 
in  this  case  may  be  the  body  of  a 
dump  truck  or  a  two-wheeled  cart  (fig. 
80)  that  can  be  coupled  to  a  tractor.  The 
pipe  grid  is  on  the  bottom  of  the  body  as 
before  (temporary  mounting  in  dump 
truck),  and  a  tarpaulin  is  used  for  a 
cover.  May  be  filled  with  a  skip  loader, 
or  conveyer  belt  from  the  mixer,  and 
emptied  after  steaming  by  tipping  up  the 
front  end  and  removing  or  tilting  the 
back  panel.  May  be  used  with  free-flow- 
ing, pressure,  or  superheated  steam, 
operating  in  all  cases  without  pressure 
in  the  bin.  Two  units  may  be  used  in  a 
continuous-batch  operation.  Advantages: 
very  efficient  use  of  steam;   simple  and 

*  One  of  the  types  considered  best  for  Cali- 
fornia conditions. 


dependable;  flexibility — may  be  ma- 
chine-filled in  soil  area,  connected  to 
boiler  elsewhere,  and  dumped  into  flat- 
or  pot-filling  equipment,  on  floor,  or  in 
bins  where  needed;  recontamination  risk 
reduced;  may  also  be  used  for  pots  and 
flats,  or  soil  in  containers;  fits  well  into 
mechanization.  Disadvantage:  some  re- 
contamination  hazard.  Best  use:  treating 
bulk  soil  for  bin  storage,  or  for  use  with 
flat-filler;  also  for  pot  and  can  growing. 
References:  Anonymous  (1952)  ;  Morris 
(1953). 

3.  The  combined  bin  and 
potting  bench  . . . 

is  a  four-wheeled  table  with  the  ends 
and  one  side  fixed,  and  one  side  re- 
movable (fig.  132).  It  is  similar  to  the 
mobile  bin  (type  2).  A  perforated  pipe 
grid  is  placed  on  the  smooth  metal  floor 
of  the  bin,  which  is  then  filled  with  soil, 
covered  with  a  tarpaulin,  brought  by  a 
tractor  to  the  boiler,  and  steamed.  The 


Fig.  80.  A  mobile  bin  (type  2)  soil  steamer,  trailer  variant,  for  stationary  bulk  soil.  Dump  trucks 
are  sometimes  fitted  with  perforated  steam  grids  to  give  larger  units  of  this  type.  A  small  portable 
steam  generator  is  shown  at  the  rear. 


[168] 


pipe  grid  may  then  be  lifted  out,  the 
vehicle  pulled  to  the  potting  site,  one 
side  removed,  and  the  potting  done 
directly  on  the  bench.  The  unit  may  be 
used  with  free-flowing,  pressure,  or  su- 
perheated steam,  operating  in  all  cases 
without  pressure  in  the  bin.  Advantages : 
efficient  use  of  steam;  simple  and  de- 
pendable; labor  saving.  Disadvantages: 
considerable  recontamination  hazard 
because  whole  load  is  exposed  to  in- 
festation from  the  potting  operation 
when  soil  is  first  used;  extreme  sanita- 
tion necessary;  fair  integration  with 
other  mechanization.  The  contamination 
hazard  could  be  minimized  by  construct- 
ing on  the  wagon  bed  a  tapered  soil  bin, 
from  the  bottom  of  which  the  soil  flows 
through  a  variable  gate  onto  the  potting 
table.  A  permanent  perforated  pipe 
steam  grid  could  be  built  in  the  bin.  The 
treated  soil  would  be  thus  protected  from 
recontamination  until  used.  Best  use: 
pot-plant  growing.  Reference:  Anony- 
mous (1954). 

*4.  The  steam  box  . . . 

is  a  wooden  or  metal  box  equipped 
with  a  hinged  lid  for  filling  with  soil, 
and  a  dump  bottom  for  emptying.  It  is 
elevated  from  the  floor,  and  is  stationary 
( or  may  be  mounted  on  wheels  for 
mobility,  if  fabricated  of  metal).  If  it  is 
made  of  wood,  resin-impregnated  marine 
plywood2  must  be  used,  as  others  will 
deteriorate  when  steamed.  The  "cold" 
edges  and  bottom  four  corners  should  be 
fitted  with  triangular  pieces  of  wood  to 
expedite  uniform  heating.  These  four 
corners  are  usually  the  coldest  spots  in 
all  batch  treating  equipment.  In  the  box 
is  a  perforated  grid  of  pipes  arranged  in 
rows  9  in.  apart  each  way;  on  the  under- 
side Vs-in.  holes  are  drilled  about  9  in. 

*  One  of  the  types  considered  best  for  Cali- 
fornia conditions. 

2  Phenolic  resin  glueline  and  coating  on  sur- 
faces; may  be  called  High  Density  Overlaid 
Plywood. 


apart.  This  arrangement  insures  that  no 
soil  particle  is  more  than  5  in.  from 
several  steam  outlets,  which  provides 
maximum  efficiency  in  distribution  of 
steam  and  speed  of  heating.  It  is  filled 
from  above,  emptied  below.  There  are 
several  methods  for  operating  the  bottom 
doors;  among  the  best  are  controlled 
lowering  by  a  chain  hoist,  rack  and 
pinion  gears,  sprockets  and  chains,  or  by 
a  cable  wound  on  a  racheted  shaft;  a 
mechanical  trip-catch  on  free-falling 
doors  may  also  be  used. 

It  may  be  made  with  a  fixed  front 
(type  4a;  figs.  79  and  81)  for  bulk  soil 
only,  or  with  the  front  panel  removable 
(type  4b;  figs.  5  and  82),  so  it  may  be 
used  for  bulk  soil  or  for  flats  and  pots, 
empty  or  filled  with  soil,  these  being 
placed  on  the  pipe  shelves.  The  units 
were  developed  by  the  authors  in  the 
Department  of  Plant  Pathology,  Univer- 
sity of  California,  Los  Angeles.  A  Los 
Angeles  company  will  fabricate  such 
boxes  on  special  order  (see  Appendix). 
When  used  for  flats,  steam  is  released 
into  only  the  bottom  layer  of  pipes,  the 
valve  closing  off  the  other  pipes.  The 
bulk-soil  type  is  useful  for  nurseries 
with  fairly  large  soil  requirements,  since 
the  box  may  be  dumped  once  an  hour, 
or  oftener  if  a  large  steam  flow  is  used. 
Continuous-batch  operation  may  be 
achieved  by  having  two  boxes  in  tandem, 
one  being  filled  while  the  other  is  steam- 
ing. Such  a  unit  was  successfully  used  on 
a  contract  basis  in  southern  California 
for  several  years,  turning  out  about  16 
cu.  yd.  per  day.  When  such  a  dual  unit 
is  coupled  to  an  automatic  flat-filler 
(Sec.  17),  an  essentially  continuous 
output  of  flats  is  obtained.  Anv  of  these 
units  may  be  used  with  free-flowing, 
pressure,  or  superheated  steam,  operat- 
ing in  all  cases  without  pressure  in  the 
box.  The  front-opening  tvpe  is  probablv 
the  best  available  for  the  small  nursery, 
because  of  its  flexibility  and  efficiency. 
These  boxes  are  probably  the  most  effi- 


[109] 


Fig.  81.  The   fixed-front   steam   box   for   sta- 
tionary  bulk  soil   (type  4a).  See  also  fig.   79. 

cient.  dependable,  and  convenient  of  the 
bulk-type  equipment,  and  fit  admirably 
into  mechanization.  For  example,  a 
Toledo,  Ohio,  nursery  is  said  to  use 
boxes  of  this  type  holding  3%  cu.  yd., 
arranged  so  that  the  soil  drops  directly 
into  dump  trucks.  Advantages :  very  effi- 
cient use  of  steam;  rapid  and  uniform 
heating  of  soil;  simple  and  fairly  inex- 
pensive to  construct;  very  dependable, 
with  few  moving  parts;  highly  adaptable 
in  the  various  forms,  fitting  well  into 
mechanization;  cannot  be  overheated 
(unless  used  with  superheated  steam)  ; 
ease  of  handling;  containers  also  treated 
(in  open-front  type  only).  Disadvan- 
tages: since  containers  are  not  treated 
(except  in  open-front  type),  this  must 
be  done  separately;  some  recontamina- 
tion  hazard  with  bulk  type,  as  soil  must 
be  handled.  Best  use:  bulk-soil  type, 
single  or  dual,  with  or  without  attached 
flat-filler,  is  excellent  for  large  nurseries. 
Open-fronl  type  nearly  ideal  for  small 
nursery.  Reference:  Roislacher  and 
Baker  (1956). 

I  I 


Fig.  82.  The  removable-front  steam  box  for 
stationary  bulk  soil,  and  for  soil  in  containers 
(type  4b).  See  also  fig.  5. 

PressureSess   Steam   from    External 
Source  Released  around  Soil 

*5.  The  Thomas  method  for 
steaming  soil  in  containers  .  • . 

should  be  more  widely  used.  The  prin- 
ciples and  methods  are  the  same  as 
described  for  the  surface  Thomas 
method  (type  13;  see  fig.  91).  Flats  or 
pots  are  stacked  on  the  concrete  floor 
with  wood  separators  between  layers,  or 
in  special  steel  racks,  and  covered  with 
a  rubberized  canvas  or  similar  material. 
The  tarp  is  held  down  by  pipes  or  sand 
bags  along  the  edges.  Steam  is  released 
under  the  cover,  which  then  acts  as  a 
steam  chamber.  The  method  may  be  used 
with  free-flowing,  pressure,  or  super- 
heated steam,  operating  in  all  cases 
without  pressure  in  the  soil.  Advantages: 
inexpensive,  simple,  efficient;  materials 
readily  available;  used  for  any  con- 
tainer, empty  or  full;  also  used  for  bulk 
soil   (type  18)  ;  adaptable  to  mechaniza- 

*  One  of  the  types  considered  best  for  Cali- 
fornia conditions. 


70  | 


tion  by  use  of  pallets  and  fork-lift  trac- 
tor. Disadvantages :  cover  may  wear  out, 
particularly  if  paper;  takes  up  work 
area  unless  a  separate  floor  is  provided. 
Best  use:  small  nursery;  larger  opera- 
tions should  use  permanent  equipment. 
Reference:  Dimock  and  Post  (1944). 

*6.  The  vault  type  . . . 

has    been    used    for    at   least   25    years, 
and  is  still  one  of  the  best  for  soil  in 
flats  and  pots.  It  consists  of  a  vault  built 
of  reinforced  concrete,  planks,  or  heavy 
resin-impregnated      marine      plywood,3 
with  a  concrete  floor  continuous  with  that 
outside  (figs.  83,  128,  and  131 ) .  A  swing- 
ing wooden  door  on  one  end  is  sealed 
around  the  edges  with  a  gasket  of  rubber 
steam  hose,  and  held  tightly  closed  by 
several    refrigerator-type    door    clamps. 
The  unit  is  not  steamtight  and  will  not 
build  up  pressure.  The  steam  will  con- 
dense on  the  soil,  with  only  slight  leak- 
age   from    the    vault,    until    the    inside 
temperatures    approach    212°    F,    when 
increasing    amounts    will    be    lost.    The 
steam  is  admitted  to  the  vault  from  a 
perforated  pipe  on  the  floor  against  the 
sides    and    back    end,    with    the    holes 
toward  the  inside  of  the  vault  and  down- 
ward on  a  45°  angle  with  the  floor.  Such 
a  vault  may  be  loaded  manually,  flats 
being    stacked    with     %-in.     separator 
strips  or  as  shown  in  figure  104,  and  pots 
set  on  top  of  each  other.  A  better  system, 
however,  is  to  pile  the  flats  or  pots  on 
pallets  and  load  these  into  the  vault  with 
a   fork-lift  tractor.   While  such   a  vault 
can  be  used  for  bulk  soil  in  large  boxes, 
it   is   less   efficient   than   the   perforated 
pipe  grid  for  this  purpose.  These  units 
may  be  built  in  any  size,  depending  on 
the    needs    of   the   nursery    and    on   the 
boiler  capacity.  Two  vaults  may  readily 
be  used  in  a  continuous-batch  operation. 
The  equipment  may  be  used  with  free- 
flowing,  pressure,  or  superheated  steam, 
operating  in  all  cases  without  pressure 
in  the  vault.  Advantages:  soil  steamed 


directly  in  containers,  reducing  both  the 
handling  after  treatment  and  the  recon- 
tamination  hazard;  quite  efficient  use  of 
steam;  convenient;  dependable,  without 
moving  parts;  quite  inexpensive  to 
build;  adaptable  to  any  type  of  con- 
tainer, empty  or  full,  and  even  bulk  soil 
(with  lowered  efficiency)  ;  fits  well  into 
a  mechanization  program;  heating  soil 
above  212°  F  is  possible  only  with 
superheated  steam.  Disadvantages:  may 
be  awkward  to  load  and  unload  man- 
ually, with  some  chance  of  getting 
burned;  if  soil  load  is  excessive  for 
boiler  capacity,  treatment  may  take 
several  hours;  poorly  adapted  for  treat- 
ing bulk  soil.  Best  use:  excellent  in 
general  growing  operations,  manually 
handled  and  of  small  size  in  small  nur- 
series, mechanically  handled  and  large 
for  bigger  ones.  Reference:  Newhall, 
Chupp,  and  Guterman  (1940,  p.  33-34). 

*7.  The  multipurpose-tank  type  .  .  . 

is  a  modification  of  the  vault  steamer 
designed  so  that  it  may  be  used  for  soil 
and  container  disinfestation,  and  also  as 
a  hot-water  tank  for  treatment  of  seeds, 
bulbs,  and  planting  stock  (Sec.  13),  as 
well  as  a  soaking  and  steaming  tank  for 
pots,  cans,  and  flats.  This  combination 
was  suggested  by  C.  E.  Scott,  California 
Agricultural  Extension  Service,  and  the 
equipment  designed  in  1940  by  H.  Gor- 
don, Department  of  Agricultural  Engi- 
neering, University  of  California,  Davis. 
It  has  been  extensively  used  by  the  De- 
partment of  Plant  Pathology,  Los  An- 
geles, for  16  years  and  has  proved  to  be 
the  "work  horse"  of  our  treating  equip- 
ment. It  consists  essentially  of  an  in- 
sulated horizontal  metal  shell,  with  a 
hinged  lid  (figs.  84  and  85).  The  lid  is 
loosely  sealed  with  a  gasket  of  rubber 
steam  hose.  Steam  is  released  through  a 
perforated  pipe  in  the  bottom  of  the  tank. 

*  One  of  the  types  considered  best  for  Cali- 
fornia conditions. 
3  See  footnote  2,  above. 


[171] 


83 


S-**c 


^^S^^^^S^S^SSSk 


^^S^r^S^S 


84 


Fig.  83.  (Top)  The  vault-type  steamer  for  soil  in  stationary  containers  (type  6).  See  also  figs. 
128  and  131.  Fig.  84.  The  multipurpose  tank  (type  7)  for  steaming  soil  in  stationary  containers, 
for  hot-water  treatment  of  planting  material,  and  for  soaking  salt  from  pots.  See  also  fig.  85. 


I   172] 


Fig.  85.  The  multipurpose  tank  (type  7)  for  steaming  soil  in  stationary  containers,  for  hot-water 
treatment  of  seeds  and  planting  material,  and  for  soaking  salt  from  pots.  Note  the  outlet  pipe  for 
circulating  water  in  left  photo,  and  pump  and  input  in  the  view  on  the  right.  The  input  pipe  is 
fitted  with  a  valve,  and  also  serves  as  the  drain  pipe.  A  thermometer  is  placed  in  the  input  line. 
The  steam  input  pipe  (white),  and  method  of  stacking  flats  are  shown.  See  also  fig.  84. 


The  unit  is  not  steamtight  and  cannot 
build  up  pressure.  It  may  be  operated 
on  free-flowing,  pressure,  or  superheated 
steam.  Into  this  container  may  be 
stacked  soil  in  flats,  cans,  or  pots,  with 
the  layers  separated  by  wood  strips.  It 
may  also  be  used  to  steam  empty  con- 
tainers prior  to  use.  The  tank  may  be 
filled  with  water  and  heated  by  passing 
steam  through  a  pipe  in  the  bottom,  with 
the  condensate  passing  out  of  the  tank, 
or  steam  may  be  released  directly  into 
the  water.  The  temperature  may  be  ac- 
curately controlled  by  adjusting  the 
steam  valve.  The  circulating  pump  re- 
moves the  water  from  the  top  at  one  end 
and  returns  it  to  the  bottom  at  the  other, 
providing  excellent  water  circulation. 
Used  in  this  way  the  unit  has  proved 
very  efficient  for  the  hot-water  treatment 
of  seeds,  corms,  and  whole  plants.  With 
or  without  the  circulating  pump  oper- 
ating, it  provides  an  excellent  tank  for 
soaking  the  salt  out  of  pots  (Sec.  4). 
Such  units  can  be  built  in  a  size  suitable 
for  the  given  nursery.  Advantages:  soil 


is  steamed  directly  in  the  containers; 
quite  efficient  use  of  steam  (it  should  be 
turned  very  low  when  condensation  has 
nearly  ceased  and  much  steam  is  escap- 
ing) ;  dependable, without  moving  parts; 
quite  inexpensive  to  build;  adaptable  to 
any  type  of  container,  empty  or  full; 
heating  soil  above  212°  F  is  possible 
only  with  superheated  steam;  extremely 
flexible,  triple-function  unit.  Disadvan- 
tages: awkward  to  load  and  unload; 
poorly  adapted  for  treating  bulk  soil; 
does  not  fit  well  into  mechanization. 
Best  use:  excellent  for  general  growing 
operations,  particularly  where  heat 
treatment  of  seeds  and  propagative  ma- 
terial is  planned;  best  suited  for  triple 
function  in  small  nurseries  and  in  experi- 
ment stations. 

8.  The  vertical-cabinet  type  .  .  . 

consists  of  a  wood  (marine  or  resin- 
impregnated  plywood4)  or  metal  cup- 
board, on  the  shelves  of  which  are  placed 
flats,  pots,  or  cans  of  soil  to  be  treated. 


4  See  footnote  2,  above. 


[173] 


Steam  is  released  into  the  bottom  of  the 
cabinet  and  condenses  on  the  cool  con- 
tents as  outlined  for  the  vault  type  (type 
6).  Apparently  this  was  first  adapted 
from  self-generating  equipment  used  for 
dairy  utensils.  The  usual  form  is  a  wall 
cabinet  with  pipe  shelves  on  which  flats 
and  pots  may  be  placed.  The  door  should 
have  a  gasket  of  rubber  steam  hose. 
Similar  to  the  vertical-cabinet  type  (type 
11;  fig.  88).  May  be  used  with  free- 
flowing,  pressure,  or  superheated  steam, 
operating  in  all  cases  without  pressure 
in  the  cabinet.  Advantages :  soil  steamed 
directly  in  the  containers;  quite  efficient 
use  of  steam;  dependable,  without  mov- 
ing parts;  inexpensive;  adaptable  to 
various  containers,  empty  or  full;  heat- 
ing soil  above  212°  F  is  possible  only 
with  superheated  steam;  soil  not  han- 
dled after  treatment.  Disadvantages :  not 
adapted  to  large  volume  or  to  bulk  soil; 
awkward  to  load  and  unload;  does  not 
fit  well  into  mechanization.  Best  use: 
small  nursery  using  flats,  pots,  or  cans, 
and  not  requiring  bulk  soil.  Reference: 
Johnson  (1930,  p.  4-5). 

Pressure  Steam  from  External 
Source  Released  around  Soil 

9.  Autoclaves  or  cannery  retorts  .  . . 

(figs.  6  and  86)  are  sometimes  used  for 
soil  treatment.  Usually  they  are  second- 
hand, as  cost  would  otherwise  be  pro- 
hibitive. When  used  at  15  lb.  steam  pres- 
sure, the  exhaust  valve  or  the  door  must 
be  left  open  for  a  time  to  displace  the 
air  with  steam.  Sometimes  operated  as 
pressureless  containers,  when  the  unit 
becomes  the  vault  type  (type  6).  Heat 
transfer  by  steam  at  15  lb.  pressure  is 
only  14  B.t.u.  per  lb.  greater  than  when 
free-flowing,  and  the  temperature  only 
37.8  F  higher  (fig.  77).  May  be  mech- 
anized l>\  stacking  containers  on  pallets 
thai  are  loaded  into  autoclave  with  a 
lork-lifi  tractor.  Two  autoclaves  could 
be  used  in  tandem  in  a  continuous-batch 


operation.  Advantages:  slightly  faster 
heating  (about  1.4  per  cent)  than  with 
flowing  steam;  soil  treated  in  contain- 
ers; efficient;  dependable,  without  mov- 
ing parts;  fits  well  into  mechanization 
program;  adaptable  to  any  type  of  con- 
tainer, full  or  empty.  Disadvantages :  re- 
quires pressure  boiler  to  operate  it;  high 
initial  cost;  difficult  to  load  and  unload, 
with  some  chance  of  getting  burned;  soil 
heated  to  unnecessarily  high  tempera- 
ture; if  air  is  not  displaced  by  steam  be- 
fore closing  the  door  very  uneven  heat- 
ing may  result.  Best  use:  general  grow- 
ing operations  in  medium  to  large 
nursery.  Reference:  Newhall,  Chupp,  and 
Guterman  (1940,  p.  33). 

Pressureless  Steam  from  Built-in 
Generator  Released  around  Soil 

Several  kinds  of  soil-treating  equip- 
ment may  be  modified  in  such  a  way 
that  water  may  be  converted  to  steam 
inside  the  unit  by  heat  supplied  from 
gas,  oil,  butane,  or  propane  flame,  or  by 
electricity.  Usually  a  metal  pan  of  water 
is  heated  until  sufficient  water  is  evapo- 
rated to  raise  the  temperature  to  180° 
to  212°  F.  To  be  effective,  therefore, 
such  units  must  contain  that  amount  of 
water,  plus  an  excess  to  avoid  boiling 
dry.  To  raise  the  soil  temperature  of  1 
cu.  ft.  of  soil  with  15  per  cent  moisture 
150  degrees  F  will  require  that  about  6V2 
lb.  (about  61/4  pints)  of  water  be  con- 
verted to  steam,  assuming  50  per  cent 
efficiency.  Equipment  of  this  sort  usually 
contains  up  to  twice  this  amount  of 
water. 

10.  Horizontal  type  with 
removable  hood  .  .  . 

is  used  by  several  nurseries  in  Califor- 
nia. It  consists  of  a  metal  pan  contain- 
ing water  underneath  a  rack  on  which 
flats  or  pots  are  stacked.  A  large  metal 
hood  is  lowered  over  the  stack  by  a  hoist, 
to  form  the  steam  chamber  (fig.  87). 
Heat  is   applied   by  gas  or  oil  burners 


[174] 


86 


g  g  *?* 


Fig.  86.  Cannery  retort  (autoclave)  for  steaming  soil  under  pressure  in  stationary  containers 
(type  9).  See  also  fig.  6.  Fig.  87.  Horizontal  type  of  steamer  with  removable  hood  (type  10). 
Steam  is  generated  from  the  water  in  the  metal  pan  below,  and  containers  of  soil  are  stacked 
on  a  wooden  frame  above  it.  Fig.  88.  The  vertical  cabinet  type  of  steamer  (type  11)  for  steam- 
ing soil  in  stationary  containers.  Steam  is  generated  from  the  water  in  the  metal  pan  below. 
Steam  may  be  released  into  the  cabinet  (type  8);  the  metal  pan  is  then  omitted. 


[175] 


under  the  pan,  or  by  electric  immersion 
heaters  in  it.  If  externally  generated 
steam  is  released  into  such  a  unit,  it  es- 
sentially becomes  a  vertical-cabinet  type 
(type  8).  Two  such  units  could  be  used 
in  a  continuous-batch  operation.  Ad- 
vantages: soil  treated  in  containers;  in- 
expensive; cannot  be  overheated;  fairly 
efficient  use  of  steam;  easy  to  load  and 
unload;  fits  fairly  well  into  small-scale 
mechanization.  Disadvantages:  not  use- 
ful for  bulk  soil;  slow  operation;  im- 
practical for  more  than  16  flats  per  load. 
Best  use:  small  nursery  having  little 
need  for  bulk  soil;  home  garden  or 
hobby  greenhouse.  Reference:  Califor- 
nia Dept.  Agr.   (1944). 

1 1.  Vertical-cabinet  type  .  .  • 

is  a  modification  of  type  8  above,  in 
which  a  metal  pan  in  the  bottom  holds 
water  which  is  heated  by  a  gas  burner 
or  electric  elements  (fig.  88).  Uses  1.2 
to  2.0  k.w.h.  per  cu.  ft.  of  soil.  In  Eng- 
land the  "saucepan"  and  "trough"  meth- 
ods are  small-volume  variants  of  this 
type.  None  of  these  types  fits  into  a 
mechanization  program.  References: 
Peterson  (1942)  ;  Newhall  (1940,  p.  19- 
25)  ;  Lawrence  and  Newell  (1950,  p.  83- 
84)  ;  Lawrence  (1956,  p.  47-49). 

12.  Modified-oil-drum  type  .  .  . 

is  sometimes  used  by  very  small  nurs- 
eries in  California.  An  oil  drum  is 
mounted  vertically  on  a  brick  base  so 
that  gas  or  oil  burners  may  be  placed 
under  it.  Inside  there  is  fitted  a  metal 
base  to  keep  the  stack  of  flats  above  the 
water  that  is  placed  in  the  bottom.  A 
wooden  lid  is  used.  Advantages:  very 
inexpensive;  soil  treated  in  containers. 
Disadvantages:  very  small  capacity;  dif- 
ficult to  load  and  unload;  does  not  fit 
into  mechanization.  Best  use:  very  small 
nursery,  home  garden,  or  hobby  green- 
house. 

13.  Horizontal-tank  type  .  .  . 

is  a  variant  of  the  multipurpose  tank 
(type  7),  apparently  little  used  in  this 


country.  Two  modifications  are  used  in 
England.  In  one  the  tank  has  water  in 
the  bottom  and  either  soil  in  bulk  or  in 
containers  supported  in  stacks  over  it; 
heat  is  applied  from  beneath  by  coal, 
oil,  or  electricity.  In  another  type,  the 
tank  is  mounted  on  wheels  for  porta- 
bility, and  has  in  the  understructure  a 
coal-fired  furnace  to  heat  the  water  in 
the  tank.  Reference:  Lawrence  and  New- 
ell (1950,  p.  85-87,  145-57). 

Dry  Source  of  Heat 

Thermal  transfer  from  dry  heaters  to 
a  stationary  soil  mass  is  effected  in  one 
of  two  ways. 

1.  If  the  soil  is  dry  there  is  slow  and 
inefficient  transmission  from  particle  to 
particle,  the  air  spaces  between  acting  as 
insulation.  It  is  necessary,  therefore,  to 
use  high  temperatures  for  long  periods 
to  get  penetration  of  heat,  and  this  usu- 
ally means  that  the  soil  is  excessively 
heated  at  the  point  of  contact,  with  char- 
ring of  the  organic  matter. 

2.  If  the  soil  is  moist,  the  soil  water 
at  the  heat  source  is  converted  to  steam, 
which  moves  outward  in  a  zone,  as  ex- 
plained in  Section  9.  It  is  evident,  how- 
ever, that  in  the  zone  where  the  moisture 
has  been  evaporated,  the  soil  will  be 
heated  to  excess,  as  in  the  case  above. 
There  is  an  outer  moving  zone  of  steam, 
followed  by  an  inner  expanding  sphe- 
roid of  drying  soil,  and  at  the  center  an 
area  of  very  high  temperatures  (not  un- 
commonly reaching  400°  to  500°  F  in 
electric  heater  types)  and  charred  or- 
ganic matter.  If  continued  long  enough, 
the  mix  would  be  entirely  desiccated  and 
perhaps  charred. 

14.  Box  with  electrical  heating 
elements  in  soil  .  .  . 

is  now  less  commonly  used  than  before. 
Also  called  the  New  York,  immersion,  or 
indirect  type;  it  has  been  known  since 
L931 .  It  consists  of  a  wooden  box  similar 
to   the   steam   box    (type  4a   above)    in 


r  i7<>  i 


which  are  mounted  special  electrical 
heating  elements  (fig.  89).  These  heavy- 
duty  immersion  heating  elements  are  of 
the  strap,  plate,  or  rod  types.  Current 
consumption  about  1  to  1.5  k.w.h.  per 
cu.  ft.  The  "cold"  edges  and  bottom 
four  corners  should  be  fitted  with  tri- 
angular pieces  of  wood  to  expedite  uni- 
form heating.  A  small  (Va  cu.  ft.)  tubu- 
lar model  with  single  central  heating 
unit  is  used  in  England  by  amateurs  and 
gardeners.  Advantages:  simple,  safe  op- 
eration; constant  power  load;  quite  ef- 
ficient; inexpensive  to  build.  Disadvan- 
tages: requires  quite  moist  soil  to  func- 
tion without  burning;  chars  organic 
matter  next  to  heating  elements;  uneven 
heating;  expensive  to  operate;  used  for 
bulk  soil  only,  with  containers  untreated; 
very  slow  heating;  does  not  fit  well  into 
mechanization.  Best  use:  in  small  nurs- 
eries with  other  means  of  treating  con- 
tainers, but  without  steam  source.  Ref- 
erences: Newhall  (1940,  p.  5-8,  14-18)  ; 
Hardy  and  Dillon,  Inc.  (1953)  ;  Brown 
and  Wakeford  (1947). 

15.  Electrode  type  with  soil  heated 
by  resistance  to  electric  current  .  .  . 

sometimes  called  the  Ohio  or  direct  type, 
is  seldom  used  in  California,  but  ap- 
parently is  in  England.  It  has  been  avail- 
able since  1921.  It  consists  of  a  box 
similar  to  the  steam  box  (type  4a),  in 
which  are  mounted  metal  electrodes 
(fig.  90).  The  current  passes  through 
the  soil  solution  between  electrodes  due 
to  the  presence  of  dissolved  salts,  gen- 
erating heat  in  the  soil  in  the  vicinity  of 
the  plates  as  a  result  of  the  resistance 
presented.  Current  consumption  varies 
from  1  to  4  k.w.h.  per  cu.  ft.  of  soil.  A 
transformer  is  required  for  best  results. 
Soil  is  dried  in  treatment.  In  England 
a  harrow-type  grid  of  rod  electrodes  has 
been  successfully  used,  with  a  trans- 
former, on  ground  beds;  this  required  3 
to  4  k.w.h.  per  cu.  ft.  to  reach  158°  F. 


Advantages:  shuts  off  automatically  at 
180°  to  200°,  as  water  boils  away  from 
electrodes;  relatively  uniform  heating. 
Disadvantages:  serious  shock  hazard; 
transformer  is  usually  necessary  and 
makes  equipment  expensive;  current 
load  varies  widely  as  soil  dries  along 
plates,  with  firmness  of  soil  packing, 
temperature,  and  with  salt  content  of 
soil;  in  some  areas  (probably  not  in 
California)  addition  of  epsom  salts  or 
potassium  nitrate  is  necessary;  difficult 
to  insulate,  particularly  in  ground  beds; 
not  adapted  to  mechanization  schedule. 
Best  use:  little  used;  suitable  only  for 
small  nursery.  References:  Canham 
(1951)  ;  Newhall  (1940,  p.  3-5)  ;  Taver- 
netti  (1935). 

16.  Box  type  with  soil  heated  by 
electrical  induction  grid  .  .  . 

has  very  rarely  been  used.  It  consists  of 
a  box  similar  to  the  steam  box  (type 
4a),  in  which  an  iron  pipe  grid  enclos- 
ing coils  of  copper  wire  is  buried  in 
soil.  Alternating  current  passes  through 
the  coils,  sets  up  currents  in  the  pipes, 
heating  them.  Advantages:  as  in  type 
14.  Disadvantages:  high  initial  cost; 
localized  heating,  as  in  14;  insulation  of 
wire  deteriorates;  not  adapted  to  mech- 
anization program.  Best  use:  limited  to 
small  nurseries.  Reference:  Newhall 
(1940,  p.  13-14). 

17.  Baking  or  burning  of  soil  .  .  . 

an  ancient  method,  is  now  obsolete  be- 
cause it  is  inefficient  and  destroys  the 
soil  organic  matter.  Since  organisms  are 
more  resistant  to  dry  than  to  moist  heat, 
the  biological  efficiency  is  low.  Because 
of  low  heat  conduction  by  soil  this 
method  is  expensive  and  inefficient.  Usu- 
ally soil  is  placed  in  containers  in  an 
oven,  but  in  England  special  brick  struc- 
tures are  used.  Not  recommended.  Ref- 
erence: Bewley  (1939,  p.  22-27). 


[177] 


STATIONARY  SOIL  MASS  IN  BENCHES  OR  BEDS 


The  preceding  seventeen  types,  as  well 
as  types  27  to  35,  treat  soil  in  bulk  or  in 
containers.  Nurserymen  frequently  must 
treat  soil  in  benches  or  beds,  and  the 
methods  now  to  be  discussed  have  been 
used  for  this  purpose.  Surface  or  pan 
types  (types  18,  19,  24)  heat  the  soil 
downward  from  the  surface,  whereas 
buried  pipes  or  tiles,  or  the  spike  or  rake 
methods  (types  20  to  23)  heat  upward 
from  the  bottom.  It  is  reasonable,  and 
is  supported  by  abundant  experience, 
that  the  buried-pipe  or  -tile  or  the  mov- 
ing-rake methods  give  the  deepest  soil 
treatments.  Where  penetration  below  8 
to  9  in.  is  not  required,  the  Thomas 
method  (type  18)  may  be  used,  but 
where  penetration  to  12  to  18  in.  or  more 
is  required,  the  buried-pipe  or  -tile  or 
moving-rake  method  is  necessary. 

Pressureless  Steam   from   External 
Source   Released   around   Soil 

18.  The  Thomas  or  surface 
method  of  steaming  .  .  . 

(fig.  91)  has  been  widely  adopted  in  the 
past  13  years  in  this  country  and  abroad. 
A  canvas  hose,  aluminum  pipe,  metal 
downspout,  perforated  pipe,  or  simply 
a  series  of  croquet  wickets  set  in  the 
soil,  is  placed  lengthwise  on  the  top  and 
center  of  the  bed  to  be  steamed.  Over 
this  is  spread  a  plastic  sheet  (Visqueen, 
Duratex,  Stericover,  Velon  Fumicover), 
rubberized  cloth,  treated  fabric  (Fiber- 
thin,  Tufedge) ,  treated  fiberglass  (Steril- 
tex),  or  paper  (Sisalkraft) .  These  may 
be  fitted  and  tied  around  glasshouse 
posts,  or  two  strips  may  be  laid  length- 
wise and  joined  at  a  wire  stretched  down 
the  line  of  posts,  by  folding  the  edges 
together  and  clipping  with  spring-type 
wooden    clothespins    (fig.    91).    Sonic   of 

*  One  of  the  types  considered  best  for  Cali- 
fornia conditions. 


the  materials  automatically  seal  them- 
selves to  the  sides  of  the  bed  with  the 
condensate ;  others  may  be  held  down  by 
2x4  timbers  (hot  metal  angle  iron  or 
pipe  may  injure  some  plastics)  placed 
on  top  of  sideboards.  Wood  strips  may 
also  be  held  by  C  clamps  against  the  ma- 
terial on  the  inside  of  the  sideboard,  but 
this  reduces  effectiveness  of  treatment  of 
sides.  Lath  may  be  tacked  over  the  ma- 
terial to  the  outside  of  the  sideboards. 
The  arrangement  should  be  nearly 
steamtight  and  arranged  so  that  it  may 
inflate  with  steam  to  a  height  of  5  to  6 
in.  Steam  should  be  turned  on  slowly  to 
avoid  blowing  off  the  cover.  Covers  may 
be  used  many  times  if  properly  cared  for. 
Plastic  covers  should  not  touch  hot  steam 
pipes  and  should  not  be  exposed  un- 
necessarily to  sunlight,  nor  stored  wet. 

These  covers  act  like  a  metal  inverted 
pan  (type  19).  The  soil  should  be 
loosened  to  the  bottom  of  the  bench  or 
the  desired  depth  of  penetration,  and  be 
in  good  planting  condition,  free  of  clods. 
The  end  of  the  bench  where  steam  is  in- 
troduced may  be  difficult  to  heat  because 
of  condensed  water  expelled  there  from 
the  pipes,  or  because  of  a  dead  air 
pocket.  The  condensate  and  moist  steam 
should  be  drained  from  the  pipes  before 
attaching  the  hose.  A  short  perforated 
side  pipe  from  the  main  at  this  point  will 
provide  steam  distribution  at  the  input 
end.  The  cover  should  be  left  on  for  30 
min.  after  a  final  temperature  of  180°  to 
212°  F  is  reached.  The  method  may  be 
used  with  free-flowing,  pressure,  or 
superheated  steam,  operating  in  all  cases 
without  pressure  in  the  soil. 

The  method  may  be  modified  to  treat 
bulk  soil  in  the  headhouse.  Pile  the  soil 
7  to  8  in.  deep  on  floor  or  workbench, 
cover  with  tarp  and  place  boards  on  the 
edges  to  hold  it  down,  then  handle  as 
above.  It  may  also  be  used  for  soil  in 


[178] 


89 


90 


Fig.  89.  Box  type  of  heater,  using  electrical  heating  elements  immersed  in  the  stationary  bulk 
soil  (type  14).  Fig.  90.  Electrode  heater,  in  which  the  stationary  bulk  soil  is  heated  by  resistance 
of  the  soil  to  passage  of  electrical  current  (type  15).  Fig.  91.  The  Thomas  or  surface  method  of 
steaming  a  bed  of  soil  (type  18).  The  same  method  may  be  used  for  steaming  soil  in  containers 
(type  5). 


[179] 


Hats  or  pots,  stacked  with  separators,  in 
the  same  way  (type  5). 

Another  variation  has  been  used  for 
ground  beds  in  lath  houses.  A  flat  frame- 
work of  1  x  4  lumber  is  placed  on  the 
ground  and  covered  with  Velon.  The 
cover  is  held  down  with  2x2  lumber 
clamped  to  the  frame.  Steam  is  released 
under  the  cover  through  a  canvas  hose. 
To  move  the  unit,  the  steam  is  shut  off 
and  the  structure  skidded  by  pulling  it 
with  attached  ropes,  so  that  it  slightly 
overlaps  the  previous  setting.  The  boards 
are  then  pressed  down  into  the  soil.  It 
is  not  necessary  to  walk  on  treated  soil 
in  this  operation. 

Advantages  of  the  Thomas  method: 
simple,  inexpensive,  with  parts  readily 
available;  light  labor  requirement;  effi- 
cient; best  practical  way  of  treating 
around  posts  and  irregular  areas;  only 
slight  danger  of  getting  burned  in  mov- 
ing sets;  permits  treating  field  soil  with- 
out walking  on  it.  Disadvantages:  ap- 
parently effectiveness  below  8  in.  is  not 
dependable;  covers  wear  out  (particu- 
larly if  of  paper)  ;  some  difficulty  in 
moving  covers  to  new  beds.  Best  use: 
excellent  for  all  bench  treatment  under 
glass  or  outdoors;  less  effective  on 
ground  beds  requiring  treatment  depth 
beyond  8  in.  References:  Dimock  and 
Post  (1944)  ;  Seeley  (1954)  ;  Ball  (1953 
[5]:  1-5;  1954). 

*  19.  The  inverted-pan  method 
of  surface  steaming  .  .  . 

has  been  in  successful  general  use  for 
about  60  years.  The  pan  (fig.  92)  is 
best  made  of  aluminum  alloy  or  other 
metal,  is  6  to  9  in.  deep,  and  of  a  size 
determined  by  the  size  of  beds,  ease  of 
handling  (by  hand  or  with  mechanical 
aids),  and  by  boiler  size.  The  pan  is 
pressed  into  the  ground  4  to  5  in.  and, 
if  high-pressure  steam  is  used,  may  re- 
quire weights  to  hold  it  down.  Soil  must 
l>c    well    worked    up.    of   good    planting 

*  One  <»f  the  types  considered  best  for  Cali- 
fornia conditions. 


moisture,  and  free  of  clods.  Tempera- 
ture may  be  brought  to  180°  to  212°  F 
at  8  to  9  in.  if  soil  is  well  worked  up,  but 
does  not  penetrate  a  firm  soil  layer.  Pan 
is  left  30  min.  after  reaching  tempera- 
ture, or  it  may  be  moved  to  next  posi- 
tion (overlapping  the  former)  and  the 
hot  soil  covered  by  rubberized  canvas 
to  hold  in  heat.  About  2.7  sq.  ft.  per  hr. 
per  boiler  horsepower  have  been  treated 
with  the  pan  method.  If  pans  are  to  be 
used  in  benches  or  beds,  both  should  be 
designed  for  the  most  efficient  size  and 
proper  fit.  Pans  may  be  used  with  free- 
flowing,  pressure,  or  superheated  steam, 
operating  in  all  cases  without  pressure  in 
the  soil.  Advantages:  simple,  inexpen- 
sive; efficient;  soil  not  handled  after 
treatment,  as  in  the  case  of  types  20  and 
21.  Disadvantages:  difficult  to  work 
around  posts  or  in  irregular  areas;  dif- 
ficult to  move  large  pans  (mechanical 
aids  are  available — see  Newhall,  Chupp, 
and  Guterman,  1940)  ;  danger  of  getting 
burned  while  moving  pans.  Best  use:  ex- 
cellent for  bench  or  bed  treatments  under 
glass  or  outdoors.  References:  Newhall, 
Chupp,  and  Guterman  (1940,  p.  24- 
30)  ;  Newhall  (1930,  p.  31-39). 

Pressureless  Steam  from   External 
Source  Released  into  Soil 

20.  The  buried-perforated- 
pipe  method  .  .  . 

a  development  from  the  Rudd  type,  en- 
joyed long  popularity  but  is  now  less 
used  because  of  the  labor  requirement. 
Modified  forms,  the  "Hoddesdon  pipe"  in 
England  and  the  "long  pipe"  in  Europe, 
are  extensively  used,  but  probably  will 
not  replace  the  simpler  Thomas  method 
here.  About  2Vi>  sq.  ft.  per  hr.  per  boiler 
horsepower  have  been  treated  by  the 
pipe  method.  Pipes  1  in.  in  diameter 
with  %-in.  holes  9  in.  apart,  are  laid  9 
in.  apart  and  9  to  15  in.  deep.  Morris 
(1954)  found  that  the  cross-sectional 
areas  of  the  pipe  should  be  1  Vi2  to  2 
limes  that  of  all  the  holes  in  it  to  insure 


[180  | 


Fig.  92.  The  inverted-pan  method  of  surface  steaming  of  soil  in  benches  or  beds  (type  19). 
This  unit  may  be  fitted  with  a  water  pan  and  electric  heating  elements,  to  make  a  self-generating 
unit  (type  24).  Fig.  93.  The  buried-perforated-pipe  method  of  deep  steaming  of  soil  in  benches 
or  beds  (type  20).  Fig.  94.  The  spike  method  of  deep  steaming  of  soil  in  benches  or  beds  (type 
21).  The  pan  covering  the  soil  surface  is  to  increase  efficiency.  Fig.  95.  Permanent  buried  tile 
method  of  deep  steaming  of  soil  in  benches  or  beds  (type  22). 


[1811 


uniform  flow  of  steam  to  all  of  them. 
The  distance  between  pipes  should  not 
exceed  the  depth  they  are  buried  by 
more  than  25  per  cent.  They  may  be 
single  or  joined  in  a  grid  framework 
(fig.  93).  Pipes  should  be  buried  in 
trenches  in  well-worked,  clod-free,  mod- 
erately moist  soil,  and  covered  with  rub- 
berized canvas.  When  the  temperature 
is  reached,  pipes  are  pulled  out  and 
moved  to  the  next  setting,  and  the  hot 


soil  covered  with  a  tarp.  One  Virginia 
grower  uses  perforated  2-in.  aluminum 
downspout  for  the  buried  grid  in  out- 
door beds.  Winch-drawn  adaptations 
called  "steam  plows"  are  used  in  Den- 
mark and  England,  which  eliminate 
digging  up  the  pipe  for  each  new  setting. 
Buried  pipes  may  be  used  with  free- 
flowing,  pressure,  or  superheated  steam, 
operating  in  all  cases  without  pressure  in 
the  soil.  Advantages:  deep  treatment  of 


Fig.  96.  The  moving-rake  method  of  deep-steaming  a  field  soil  (type  23).  The  rake  is  pulled 
by  a  winch  at  the  end  of  the  field,  the  slanted  blades  penetrating  to  14-in.  depth.  Fig.  97. 
Device  for  heating  water  with  steam  as  it  is  injected  into  the  soil  (type  25). 


[182] 


soil:  very  efficient  use  of  steam.  Disad- 
vantages: high  labor  cost  in  burying  and 
digging  up  pipes:  soil  must  be  handled 
after  treatment,  with  recontamination 
hazard:  danger  of  getting  burned  while 
moving  grid.  Best  use:  ground  beds  re- 
quiring deep  penetration.  References: 
Xewhall.  Chupp,  and  Guterman  (1940, 
p.  21-24);  Morris  (1954,  p.  11-13); 
Bewley  1 1939,  p.  4-11  I  :  Xewhall  (1930. 
p.  26-29);  Schmitz  (1954);  Coates 
(1954);  Hansen  (1953-54);  Lawrence 
(1956,  p.  119-20). 

21.  The   harrow,   spike,  or 
rake  method  .  .  . 

is  now  little  used.  It  consists  of  a  risid 
pipe  frame  with  vertical  teeth  on  the 
lower  side,  which  are  plunged  into  soil. 
Steam  is  released  through  holes  near  tip 
of  each  tooth.  In  England  this  is 
mounted  in  a  steam  pan  to  reduce  steam 
loss  (fig.  94).  Morris  (1954)  found  that 
the  space  between  pipes  should  not  ex- 
ceed the  depth  of  steaming  by  more  than 
about  25  per  cent,  and  that  the  spacing 
of  spikes  along  the  pipe  should  about 
equal  the  depth  of  treatment.  The  pipe 
cross-sectional  area  should  be  IV2  to  2 
times  the  area  of  the  holes  fed  bv  it  to 
insure  uniform  distribution  of  steam.  A 
single  pipe  with  vertical  teeth,  called  a 
comb  type,  is  also  used  in  England.  May 
be  used  with  free-flowing,  pressure,  or 
superheated  steam.  Advantages:  rapid 
and  easy  to  use:  inexpensive;  fairly  ef- 
ficient use  of  steam,  particularly  when 
enclosed  by  a  pan.  Disadvantages: 
serious  steam  "blow  out"  along  spikes; 
holes  plug  with  soil:  awkward  to  move: 
danger  of  getting  burned  while  moving 
grid.  Xo  longer  recommended.  Ref- 
erences: Xewhall  (1930,  p.  29-31); 
Bewley  (1939,  p.  11-12.  figs.  6-7): 
Lawrence  (1956,  p.  109-10). 

22.  Permanent  buried-tile 
method . . . 

has  clay  drain  tiles  buried  end  to  end 
in  ground  beds  13  to  16  in.  deep  and  in 


rows  18  in.  apart  1  fig.  95).  Tile  is  left 
permanently  in  place;  used  for  deep 
steaming,  as  well  as  drainage,  leaching, 
and  subirrigation.  Tiles  may  be  placed 
in  benches  for  steaming  and  then  re- 
moved, but  this  is  too  laborious,  and 
exposes  the  treated  soil  to  handling.  Soil 
is  covered  with  rubberized  canvas  dur- 
ing steaming.  About  1.3  sq.  ft.  per  boiler 
horsepower  per  hr.  have  been  treated. 
The  connecting  hoses  should  be  removed 
when  the  steam  is  shut  off,  to  prevent 
mud  being  sucked  into  the  line  as  the 
steam  condenses.  May  be  used  with  free- 
flowing,  pressure,  or  superheated  steam, 
operating  in  all  cases  without  pressure  in 
the  soil.  Advantages:  triple-function 
permanent  installation;  gives  deep  soil 
treatment.  Disadvantages:  very  high 
initial  cost;  laborious  installation;  soil 
profiles  disturbed;  tiles  must  be  reset 
after  several  years  to  remain  functional. 
Best  use:  permanent  ground  beds  under 
glass  or  lath.  Reference: Xewhall,  Chupp, 
and  Guterman   1 1940,  p.  11-21). 

23.  The  moving-rake  method  . . . 

has  recently  been  introduced  in  Florida 
for  use  on  outdoor  beds.  The  rake  con- 
sists of  a  4-in.  header  121//o  ft.  long,  on 
which  are  mounted,  at  10-in.  intervals, 
blades  16  in.  long,  set  on  a  20°  forward 
angle  (fig.  96).  Descending  immediately 
behind  each  blade,  and  bent  to  trail  14 
in.  to  the  rear  at  blade  depth,  is  a  12^n- 
steam  pipe  connected  to  the  header.  As 
the  unit  is  pulled  steadily  forward  by  a 
motor-driven  winch,  the  blades  dig  in. 
Forward  progress  is  at  about  25  ft.  per 
hr..  treating  about  320  sq.  ft.  (370  cu. 
ft. )  per  hr.,  nearly  3  sq.  ft.  per  boiler 
horsepower.  A  trailing  canvas  skirt 
covers  the  treated  soil  for  about  an  hour, 
maintaining  the  temperature  to  a  depth 
of  14  in.  An  adequate  stationary,  high- 
pressure  boiler  provides  the  steam, 
which  is  carried,  preferably  over  the 
untreated  soil  ahead  of  the  unit,  by  a 
2-in.  steam  hose.  Could  also  be  used  with 
free-flowing    or    superheated    steam    or 


[183] 


with  steam-air  mixtures.  This  unique 
method  has  many  possibilities  of  devel- 
opment. It  is  suggestive  of  the  winch- 
drawn  "long-pipe"  method  (a  buried- 
perforated-pipe  type;  type  20)  used  in 
Denmark  and  England.  Advantages: 
deep  treatment  of  field  soil  much  easier 
than  with  buried  perforated  pipe;  very 
efficient  use  of  steam;  cost  per  acre  ap- 
parently no  greater  than  for  some  fungi- 
cidal treatments.  Disadvantages:  initial 
cost  of  equipment;  large  steam  boiler 
required;  has  moving  parts  to  be  main- 
tained; slow  operation.  Best  use:  field 
beds  for  high-valuation  crops.  Refer- 
ences: Ball  (1955);  Coates  (1954); 
Anonymous  (1955,  1956,  1957);  Web- 
ber (1956). 

Pressureless  Steam  from  Built-in 
Generator  Released  around  Soil 

24.  The  electric-inverted-pan 
method  of  surface  steaming  . . . 

is  a  steam  pan  (type  19,  fig.  92)  with  an 
enclosed  tray  of  water  which  is  boiled 
by  electric  heaters.  Uses  about  1.8  k.w.h. 
per  cu.  ft.  of  soil.  Advantages:  much  as 
for  type  19.  Disadvantages :  messy  opera- 
tion; high  cost  of  electricity;  limited  to 
small  operations  because  of  power  load; 
expensive  heavy  wiring  required  in  glass- 
house; overheats  soil  under  electric 
elements.  Best  use:  small  nursery  using 
no  bulk  soil  or  containers,  and  without 
a  steam  source.  Reference:  Newhall 
(1940,  p.  25-30). 

Hot-Water  Drench  of 
Propagating  Sand 

Hot  water  transfers  a  maximum  of 
only  152  B.t.u.  per  lb.  to  soil  at  60°  F, 
whereas  steam  yields  970  B.t.u.  per  lb. 
at  212°.  Thus,  at  least  V/2  to  2  gal.  of 
water  per  cu.  ft.  of  dry  sand  is  needed  in 
order  to  raise  the  temperature  from  60° 
to   180°. 

Hot-water  drenches  of  propagating 
sand  leach  out  accumulated  salts  and,  if 
continued  until  the  sand  reaches  180°  F 
and   remains  at  that  temperature  for  30 


minutes,  will  destroy  pathogens.  If  the 
necessary  quantity  of  water  to  do  this 
is  troublesome,  the  sand  may  be  leached 
and  then  treated  with  steam. 

25.  Equipment  for  converting 
steam  to  hot  water  . . . 

as  it  is  applied  to  the  bed  is  available 
(fig.  97),  or  hot  water  directly  from  a 
hot-water  boiler  may  be  used.  Advan- 
tages :  method  is  useful  when  only  a  hot- 
water  boiler  is  available;  leaches  sand 
of  soluble  salts;  may  be  used  where 
steaming  gives  toxic  effect.  Disadvan- 
tages: extremely  messy;  not  entirely  de- 
pendable; if  used  on  soil,  may  puddle  it; 
not  used  on  ground  beds  unless  very 
well  drained.  Best  use:  sand  in  propagat- 
ing benches.  Reference:  Ball  (1942, 
p.  12-16,  19,  23-25). 

Combined  Steam  and 
Formaldehyde  Vapor 

26.  Equipment  to  volatilize  water 
and  formaldehyde  . . . 

was  described  19  years  ago,  but  has  been 
little  tested.  It  is  possible  to  drive  formal- 
dehyde as  deeply  into  soil  as  the  steam, 
with  increased  effectiveness  or  shorter 
required  treatment  time  and  reduced 
cost  of  treatment.  Most  seeds  may  be 
sown  within  24  hr.,  and  the  necessity  of 
seed  treatment  is  reduced  because  of  the 
slight  soil  residue.  Formaldehyde  is 
added  to  water  at  rate  of  1  pint  per  40 
to  80  gal.  (0.4  to  0.2  fl.  oz.  per  gal.)  in- 
jected into  flash-type  boiler  or  into  the 
steam  line,  and  the  vapor  passed  into  200 
sq.  ft.  of  soil;  with  steam  pan  this  will 
penentrate  to  10  in.  depth.  Growers  near 
Toledo,   Ohio,   have   injected   formalde- 


CAUTION: 

Many 

of 

the  < 

:hemicals 

mentioned 

in  this 

manual 

are 

poi- 

sonous   and 

may 

be 

harmful. 

The 

user  should 

carefu 

lly 

Follow  the 

pre- 

cautions  on 

the   1 

abe 

Is  of 

the 

con- 

tainers. 

184] 


hyde  into  the  steam  line  leading  to  per- 
manent buried  tiles  in  ground  beds.  They 
reported  decreased  time  and  cost,  and 
increased  efficiency  over  steam  alone. 
This  system  has  many  possibilities,  and 
should  be  further  explored.  The  prin- 
ciple involved  is  discussed  in  Section  9. 
Advantages:  cheaper  treatment  than 
steam  alone.  Disadvantages:  cannot  be 
used  near  living  plants;   may  delay  use 


of  soil;  special  boiler  or  accessories  re- 
quired. Best  use:  outdoor  ground  beds, 
or  in  houses  completely  emptied  of 
plants;  much  experimental  work  needed. 
Reference:  Beachley  (1937);  Anony- 
mous (1940). 

More  recently  Thomas  began  studies 
on  the  combined  action  of  methyl  bro- 
mide and  steam  against  nematodes. 
Reference:  Thomas  (1954). 


MOVING  SOIL  MASS  IN  CONTINUOUS  OUTPUT 


Because  this  type  of  treatment  equip- 
ment applies  heat  to  a  moving  soil  mass, 
it  is  possible  to  terminate  the  process  at 
any  desired  temperature  by  varying  the 
heat  input  (through  controlling  the 
steam,  electricity,  gas,  or  oil)  or  the  time 
of  exposure  (through  regulation  of  the 
speed  with  which  soil  is  moved).  The 
principal  disadvantages  also  arise  from 
this  feature:  power  is  required  for  the 
soil  movement;  higher  cost  initially  and 
in  maintenance  of  moving  equipment; 
greater  risk  of  breakdown  due  to  mov- 
ing parts;  lower  heat-transfer  efficiency 
because  of  losses  to  air  during  move- 
ment. 

Pressureless  Steam  from  External 
Source  Released  into  Soil 

In  this  type  of  equipment  the  steam 
condenses  more  or  less  uniformly  on  the 
soil  particles  as  they  are  tumbled  about 
until  their  temperature  reaches  212°  F. 
From  that  point  on  the  steam  simply 
escapes  to  the  surrounding  air,  and  the 
soil  temperature  is  not  raised  above 
212°  if  continued  longer. 

27.  The  continuous-knife-injector 
type  for  flats  . . . 

in  which  flats  of  soil  are  pulled  under 
the  injecting  knives,  was  designed  by  the 
Department  of  Agricultural  Engineering, 
University  of  California.  Davis.  Though 


not  much  used,  this  represents  an  inter- 
esting new  approach  to  soil  steaming 
(fig.  98) .  The  knives  slice  the  soil  in  one 
direction  as  the  flats  are  pulled  under 
them,  then  a  second  set  cut  across  the 
flat  as  it  is  drawn  at  right  angles.  Power 
is  supplied  by  moving  belts  driven  by  an 
electric  motor.  Advantages:  soil  treated 
in  containers;  continuous  operation; 
uses  efficient  external  steam  source; 
fairly  efficient  use  of  steam.  Disadvan- 
tages: small  output;  expensive  to  build; 
power  cost  and  mechanical  upkeep  of 
moving  parts;  cannot  be  used  for  bulk 
soil;  probably  insufficient  soil  volume 
for  mechanized  schedule.  Best  use: 
small  bedding-plant  nursery. 

28.  The  horizontal-rotating- 
drum  type  .  .  . 

with  steam  released  into  the  soil  mass  by 
knife  injectors  is  a  possible  variant  of 
type  30.  The  drum  rotates  on  four  rollers 
and  is  driven  by  a  large  sprocket  and 
chain  much  as  in  type  30;  in  fact  the 
basic  machine  of  that  type  (available 
without  burner)  could  well  be  modified 
for  this.  Soil  is  fed  in  through  a  hopper 
at  one  end,  is  carried  through  the  ro- 
tating drum  at  a  rate  controlled  bv  the 
adjustable  slope.  In  the  center  of  the 
drum  is  placed  a  steam  pipe  on  which 
are  welded  at  right  angles,  10  to  12  in. 
apart,    flat    hollow    knives    of    sufficient 


[185] 


Fig.  98.  The  continuous  knife-injector  steamer  for  moving  flats  of  soil  (type  27).  Fig.  99.  The 
horizontal  rotating-drum  type  of  steamer  with  knife  injectors  (type  28).  A  continuous  flow  of  bulk 
soil  passes  through  the  drum  and  into  flats  or  other  containers. 


[186] 


length  to  reach  nearly  down  to  the  drum 
(fig.  99).  These  slice  through  the  tum- 
bling soil  and  inject  steam  into  it.  The 
steam  pipe  is  closed  and  welded  to  the 
hopper  at  the  input  end,  and  the  open 
end  is  fastened  to  the  frame  at  the  other 
end  of  the  drum.  The  steam  is  obtained 
from  an  external  boiler,  and  may  be  of 
the  free-flowing,  pressure,  or  super- 
heated type.  Some  concrete  mixers  used 
for  mixing  nursery  soils  might  be  fixed 
with  similar  steam  pipes.  Advantages: 
continuous  operation;  heat  can  be  con- 
trolled to  any  level  above  160°  F  de- 
sired, by  varying  time  in  drum;  steam 
is  the  heat  source;  may  be  used  for 
simultaneous  treating  and  mixing  of 
soil;  fits  well  into  mechanization  sched- 
ule. Disadvantages:  containers  not 
treated;  initially  expensive;  power  cost 
and  mechanical  upkeep  of  moving  parts. 
Best  use:  if  different  sizes  were  available 
would  be  useful  in  many  nurseries  re- 
quiring bulk  soil. 

Pressureless  Steam  from   External 
Source  Released  around  Soil 

*29.  The  rotating-screw  type  .  .  . 

for  propulsion  of  soil  through  a  pipe 
into  which  steam  is  injected.  A  com- 
mercial unit  of  this  type  is  available  in 
California.  Gas  or  butane  burners  gener- 
ate steam  in  a  tank  beneath  the  soil  pipe ; 
the  steam  bathes  the  pipe  and  is  intro- 
duced into  it  through  holes  in  the  input 
end  of  the  shell.  The  soil  is  propelled 
through  the  pipe  by  a  screw  rotated  by 
an  electric  motor  (fig.  100).  Advan- 
tages: continuous  operation;  steam  is 
heat  source;  degree  of  soil  heating  con- 
trollable; fits  fairly  well  into  mechaniza- 
tion. Disadvantages:  containers  not 
treated;  initially  expensive;  power  cost 
and  mechanical  upkeep  of  moving  parts; 
propelling  screw  may  wear  badly.  Best 
use:  general  nursery  use  for  providing 

:,:  One  of  the  types  considered  best  for  Cali- 
fornia conditions. 


bulk       soil. 
(1953). 


Reference:       Anonymous 


Dry  Source  of  Heat 

In  equipment  with  a  moving  soil  mass 
and  dry  heaters  the  soil  particles  are 
tumbled  about  so  that  they  are  heated 
uniformly  by  direct  transmission  during 
contact  with  the  heating  element.  The 
heating  is  uniform  through  the  mass, 
whether  the  soil  is  moist  or  dry.  With 
dry  soil  there  is  some  transmission  from 
particle  to  particle  when  not  in  contact 
with  the  heat  source,  and  this  makes  for 
uniformity  through  the  mass.  In  moist 
soil  there  is  the  additional  heat  transfer 
by  steam  produced  from  water  films 
around  particles  in  contact  with  the  heat 
source.  This  steam  condenses  on  cooler 
particles,  heating  them  uniformly.  When 
all  particles  reach  212°  F,  the  steam 
escapes  to  the  surrounding  air;  if  long 
enough  continued  the  soil  is  desiccated. 
With  either  situation  there  is  no  danger 
of  charring  organic  matter  unless  treat- 
ment is  continued  beyond  212°. 

30.  The  horizontal  rotating  drum 

with  internal  blowtorch  .  .  . 

also  called  the  flash-flame  pasteurizer,  is 
used  in  the  eastern  states.  It  consists  of 
a  rotating  drum  of  adjustable  slope,  into 
which  soil  is  thrown  at  the  high  end, 
coming  out  at  the  low  end.  Into  the  drum 
from  the  low  end  is  introduced  a  flame 
from  a  large  kerosene  blowtorch,  which 
heats  the  soil  (fig.  101) .  The  commercial 
unit  is  said  to  use  2%  to  6  gal.  of  kero- 
sene per  hr.  and  to  turn  out  about  2  cu. 
yd.  of  soil  per  hr.  at  175°  to  190°  F.  A 
possible  modification  of  this  equipment 
(see  type  28)  answers  most  of  the  dis- 
advantages. Advantages:  light  weight, 
portable,  convenient;  continuous  opera- 
tion; temperature  of  soil  controllable  by 
varying  time  in  drum;  fits  fairly  well 
into  mechanization  program.  Disadvan- 
tages: containers  not  treated;  kerosene 
flame  directed  into  soil  may  leave  an 
oily  residue  which  is  injurious  to  some 


[187] 


?>>>»)>}  >>>>>>>>>>>>>>>>>>>>>>>i{kM 


—  W 


^  vl  ^c  c  c  u  c  k  j^nr 


*M***y        *M****p      M»**M    W^M    WWI¥ 


a 


^ 


^^^ 


100 


Fig.  100.  The  rotating-screw  type  of  steamer,  continuous  bulk  output  and  self-generating  (type 
29).  The  steam  prevents  overheating  in  the  tube,  and  is  injected  into  the  unit  at  the  soil-input 
end.  Fig.  101.  The  horizontal  rotating  drum  heater  with  internal  blow  torch  (type  30).  The  bulk 
soil  is  thrown  in  at  one  end,  comes  out  continuously  at  the  torch  end. 


188  1 


102 


&m_ 


^^ 


103 


104 


Fig.  102.  The  electric  hot-plate  soil  heater  giving  a  continuous  output  of  bulk  soil  (type  32). 
Fig.  103.  The  rotating  screw  type  of  electric  soil  heater  (type  33).  A  continuous  flow  of  bulk 
soil  is  supplied.  Electric  heating  elements  are  wound  around  the  tube.  Fig.  104.  Method  of  stack- 
ing flats  of  soil  to  permit  unrestricted  steam  flow  without  the  use  of  separator  strips. 


[189] 


plants  (this  might  be  avoided  by  using  a 
gas  flame)  ;  power  cost  and  mechanical 
upkeep  of  moving  parts;  dries  soil.  Best 
use:  small  nursery  using  bulk  soil  and 
having  some  means  for  treating  contain- 
ers; should  first  check  for  possible  resi- 
dual toxicity  to  the  crop  bsing  grown. 
Reference:  Newhall  and  Schroeder 
(1951). 

31.  The  horizontal  rotating  drum 
with  external  flame  heat .  .  . 

has  been  little  used.  It  consists  of  a 
rotary  drum  (sometimes  an  oil  drum 
with  the  ends  removed),  through  which 
the  soil  moves;  heat  is  applied  as  gas  or 
oil  flame  to  outside  of  drum.  Advan- 
tages: continuous  operation;  tempera- 
ture of  soil  controllable  by  varying  time 
in  drum;  moderate  initial  cost;  may  be 
used  for  simultaneous  mixing  and  treat- 
ing of  soil;  fits  fairly  well  into  mechani- 
zation. Disadvantages:  containers  not 
treated;  power  cost  and  mechanical  up- 
keep of  moving  parts;  dries  soil;  small 
capacity;  heat  torsion  of  drum.  Best 
use:  small  nursery  needing  bulk  soil  and 
having  means  of  treating  containers. 

32.  The  electric-hot-plate  type  .  .  . 

has  been  used  in  New  York,  where  it  is 
called  the  Hutchings  type.  Soil  is  pro- 
pelled by  a  chain  drive  in  a  thin  layer 
along  an  elongated  electric  hot  plate; 
it  is  fed  into  a  hopper  at  one  end  and 
drops  out  at  the  other  (fig.  102).  Uses 
about  1.5  k.w.h.  per  cu.  ft.  of  soil.  In- 
frared lights  have  been  used  to  impart 
additional  heat  to  the  soil  moving  on  the 
plate;  they  would  not  provide  sufficient 
heat  by  themselves  for  the  purpose.  Dia- 
thermy has  been  suggested,  but  the 
energy  available  is  also  insufficient  for 
this  purpose.  Advantages:  continuous 
operation;  uniform,  controllable,  rapid 
heating.  Disadvantages:  dries  soil;  con- 


tainers not  heated;  small  capacity;  high 
cost  of  electric  power  (but  could  be  op- 
erated with  gas  burners)  ;  power  cost 
and  mechanical  upkeep  of  moving  parts; 
no  stones  or  lumps  can  be  tolerated; 
capacity  too  low  for  effectively  mecha- 
nized nursery.  Best  use:  small  nursery 
requiring  only  bulk  soil  and  having 
means  of  treating  containers.  Reference: 
Newhall  (1940,  p.  30-32). 

33.  The  rotating-screw  type 
with  electric  heat  .  .  . 

has  been  little  used.  It  consists  of  a  4-in. 
pipe  6  ft.  long  wound  on  the  outside  with 
electric  heating  elements  and  housed  in 
an  insulating  shell  (fig.  103).  A  revolv- 
ing screw  forces  the  soil  from  the  hopper 
at  one  end  through  the  tube  and  out  at 
the  other  end,  about  2  min.  being  re- 
quired. Uses  about  1  k.w.h.  per  cu.  ft.  of 
soil.  Advantages:  continuous  operation; 
uniform  heating  of  soil;  efficient  use  of 
heat;  temperature  controllable  by  chang- 
ing speed  of  screw;  moderate  initial  cost. 
Disadvantages:  soil  containers  not 
treated;  power  cost  and  mechanical  up- 
keep of  moving  parts;  sticks  and  stones 
must  be  removed  from  soil;  screw 
wears  badly;  high  cost  of  electric  power; 
necessity  of  power  wiring;  dries  soil; 
small  capacity  (4  to  7  cu.  ft.  per  hr.) 
limits  use  in  mechanization  program. 
Best  use:  small  nursery  using  bulk  soil 
and  having  means  of  treating  contain- 
ers. Reference:  Tavernetti  (1942). 

34.  The  rotating-screw  type 
with  external  gas  heat  .  .  . 

has  been  little  used  although  a  com- 
mercial unit  was  available  for  a  time. 
The  pipe,  through  which  the  screw 
forced  the  soil,  was  heated  by  a  gas  or 
oil  flame.  Advantages,  disadvantages, 
and  use  as  for  type  33;  heat  produced 
torsion  of  equipment. 


90 


MOVING  SOIL  MASS  TREATED  IN  BATCHES 


Dry  Source  of  Heat 

35.  The  horizontal  rotating  drum 
with  external  flame  heat  .  .  . 

has  recently  been  introduced  as  a  com- 
mercial unit  in  the  eastern  states.  It  con- 
sists of  a  drum  which  holds  about  l1/^ 
cu.  ft.  of  soil.  Soil  is  tumbled  during 
heating,  and  the  unit  shuts  off  when  soil 
is  heated.  Uses  bottled  gas  for  fuel,  and 
treats  about  %  cu.  yd.  of  soil  per  hr. 


at  180°  F.  Fuel  cost  said  to  be  about 
$1.10  per  cu.  yd.  Advantages:  final  soil 
temperature  automatically  controlled; 
simultaneously  mixes  and  treats  soil. 
Disadvantages:  containers  not  treated: 
power  cost  and  mechanical  upkeep  of 
moving  parts;  small  capacity.  Best  use: 
small  nursery  that  has  means  of  treating 
containers.  Reference:  Tarrant  Mfg.  Co. 
(1955). 


EQUIPMENT  FOR  GENERATING  AND  DISTRIBUTING  STEAM 


Certain  aspects  of  the  generation  and 
distribution  of  steam  having  special  ap- 
plication to  its  use  in  soil  treatment  are 
here  discussed.  For  more  detailed  in- 
formation consult  standard  reference 
books  on  the  subject,  or  a  heating  engi- 
neer. Some  of  the  principles  involved 
should  be  understood  by  the  grower  for 
maximum  results. 

Types  of  Steam-Generating 
Equipment 

There  are  many  kinds  of  boilers  that 
may  be  used  for  this  purpose,  but  we 
shall  here  consider  some  of  their  general 
characteristics  rather  than  specific  types. 

High-pressure  versus  very 
low-pressure  steam 

It  was  customary  up  to  about  the  last 
decade  to  use  high-pressure  steam  for 
soil  treatment,  and  many  growers  even 
considered  this  to  be  essential  for  suc- 
cess. Now,  however,  it  is  recognized 
that:  (1)  such  steam  expands  and  the 
pressure  is  lost  when  released  into  soil, 
the  temperature  dropping  to  essentially 
that  of  free-flowing  steam;  (2)  steam  at 
80  pounds  transfers  only  36  B.t.u.  (about 
3.7  per  cent)  more  heat  per  pound  than 
does  free-flowing  steam  and  is,  therefore, 


not  a  significantly  more  effective  me- 
dium of  heat  exchange  (fig.  77);  (3) 
the  principal  benefits  from  high-pressure 
steam  are  the  faster  distribution  through 
the  pipes  (see  below),  and  the  evapora- 
tion of  droplets  of  water  through  ex- 
pansion of  the  steam  during  the  drop  in 
pressure  along  the  distribution  line;  (4) 
the  use  of  larger  distribution  mains 
achieves  the  same  improved  flow  without 
the  need  of  pressure.  From  these  con- 
siderations and  from  the  experience  of 
many  growers  has  developed  the  present 
more  economical  practice,  both  in  this 
country  and  abroad,  of  using  low-pres- 
sure boilers  with  well-insulated  mains  of 
suitable  size  and  the  shortest  length  pos- 
sible. 

There  are  other  reasons  for  using  low- 
pressure  equipment.  Because  such  boil- 
ers do  not  require  heavy  pressure-resist- 
ing shells  or  pipes,  they  are  less  expen- 
sive and  lighter  in  weight.  The  distribu- 
tion lines  may  also  be  of  lighter  con- 
struction I  although  larger  in  diameter), 
and  there  is  less  difficulty  in  making  the 
system  steamtight.  Water  may  be  fed  di- 
rectly  into  low-pressure  boilers  from 
water  mains,  and  controlled  automati- 
cally by  a  float  valve.  If  the  boiler  op- 
erates   at    above    15    pounds'    pressure 


[191] 


there  may  be  operational  restrictions  im- 
posed in  some  localities,  and  the  boiler 
insurance  is  somewhat  more  costly.  Be- 
cause of  the  higher  temperature  of  steam 
under  high  pressure  (323.9°  F  at  80 
lb.),  there  is  more  heat  loss  in  the  lines 
than  at  lower  pressures  (227.1°  F  at  5 
lb.),  and  greater  insulation  is  required. 
In  some  cases,  low-pressure  steam  is 
superheated  to  attain  greater  heat  ex- 
change (47  B.t.u.  more  per  100°  F;  see 
fig.  77).  Boiler  costs  may  be  about  the 
same  or  a  little  less  than  for  high-pres- 
sure steam.  The  lines  must  be  larger  and, 
because  of  the  higher  temperature,  well 
insulated  and  as  short  as  possible. 
Furthermore,  such  boilers  are  not  used 
for  heating  glasshouses.  For  these  rea- 
sons, superheat  boilers  are  used  spe- 
cifically for  soil  treatment.  They  may  be 
permanently  located  at  the  site  of  treat- 
ment or  may  be  portable,  thus  decreas- 
ing the  length  of  main. 

Regular  boilers  versus 
"flash"  steamers 

The  so-called  "package"  and  "flash" 
steamers  are  small  portable  units  that 
include  both  boiler  and  burner,  and  de- 
velop steam  in  a  short  period  of  time. 
Generally  they  operate  as  free-flowing 
units,  water  being  injected  into  one  end 
of  a  pipe  coil  in  a  firebox,  and  steam 
coming  out  the  other  at  very  low  pres- 
sure. The  volume  of  steam  may  be  large 
or  small,  according  to  the  size  of  the 
unit.  Types  with  a  steam  dome  that 
collects  and  returns  some  of  the  en- 
trained water  droplets  are  to  be  pre- 
ferred to  those  in  which  the  steam  comes 
directly  from  the  end  of  the  generating 
pipe,  since  they  supply  drier  steam. 
Types  that  use  part  of  the  water  in  an 
external  jacket  may  be  more  efficient  in 
the  use  of  heat.  Such  steamers  are  excel- 
lent in  small  nurseries  without  other 
sources  of  steam.  The  several  types 
presently  on  the  market  may  be  operated 
on  gas,  oil,  butane,  propane,  or  elec- 
tricity    (see    Apnendix).    They    are    de- 


signed for  continuous  output,  and  are 
not  used  in  return-type  steam  heating 
systems.  In  southern  California  they  are 
subject  to  inefficiency  due  to  rapid  scal- 
ing of  the  steam  pipes  from  the  high  salt 
content  of  some  water,  particularly  be- 
cause scale  is  collected  from  the  large 
volume  of  new  (rather  than  recirculated) 
water  that  is  heated.  In  areas  where  scale 
is  troublesome  it  may  be  desirable  to  use 
a  water  softener  (Sec.  4).  The  "flash" 
steamers  are  initially  much  less  expen- 
sive than  regular  types,  but  their  useful 
life  is  generally  less. 

There  are  many  types  of  boilers  on 
the  market,  both  new  and  secondhand. 
These  range  from  cast-iron  sectional  and 
steel  fire-tube  boilers  for  small  opera- 
tions, to  steel  water-tube  boilers  for  large 
installations.  They  may  be  vertical  or 
horizontal  types.  They  are  usually  in- 
ternally fired  (for  example,  steel  firebox 
and  marine  types),  but  may  require 
construction  of  an  expensive  external 
brick  firebox.  Most  of  this  type  give  long 
dependable  service  in  supplying  steam 
both  for  soil  treatment  and  heating  glass- 
houses. Wherever  practical  such  boilers 
are  preferable  to  less  expensive  package 
units.  With  regular  boilers,  as  with  the 
"flash"  steamers,  the  problem  of  scaling 
is  increased  when  steam  is  bled  from  the 
system  for  soil  treatment  rather  than  re- 
circulated, as  in  a  heating  circuit. 

It  is  possible  to  modify  some  (but  not 
all)  hot-water  boilers  so  that  they  will 
generate  steam  for  soil  treatment.  An 
automatic  water-level  feeder,  a  15-pound 
safety  valve,  a  water  gauge,  and  a  series 
of  valves  are  necessary.  The  water  level 
is  lowered,  and  the  upper  part  of  the 
boiler  serves  as  a  steam  dome,  or  a 
separate  steam  drum  may  be  attached. 
With  cast-iron  boilers,  the  injected  water 
should  be  preheated.  Consult  a  heating 
engineer  for  any  particular  installation. 
It  is  usually  better  to  run  separate  lines 
for  the  steam  than  to  try  to  use  the  hot- 
water  mains. 


[192] 


Stationary  versus 
portable  units 

Some  types  (for  example,  bricked-in 
boilers)  are  completely  nonportable. 
Most  other  types  may  be  portable  in  the 
smaller  sizes,  but  not  in  the  larger. 
Finally,  the  small  "package"  steamers 
referred  to  above  are  designed  for  port- 
ability. It  is  generally  most  convenient 
to  restrict  the  use  of  portable  boilers  to 
small  operations,  or  to  divide  the  large 
job  up  into  small  parts  that  can  be  han- 
dled by  such  equipment.  Both  types  of 
operation  have  a  definite  place  in  the 
California  nursery  industry. 

Aside  from  the  low  initial  cost,  there 
are  other  potential  advantages  in  a 
portable  unit.  With  such  a  unit,  the 
large  boilers  need  not  be  fired  up  for  a 
small  soil-treatment  job  during  warm 
weather.  The  steam  is  generated  at  the 
job  rather  than  being  conducted  there 
through  mains  (with  loss  of  steam,  and 
increased  steam  condensation  in  the 
process),  or  than  taking  the  job  to  the 
steam  source. 

In  England  it  is  possible  to  rent 
boilers  for  soil  steaming.  Custom  steam- 
ing is  done  in  England,  Europe,  and  in 
New  York  state.  Some  English  growers 
have  also  grouped  together  to  purchase 
a  boiler,  much  as  farmers  in  the  eastern 
part  of  this  country  have  formed  "spray 
rings"  to  buy  equipment  for  their  or- 
chard or  potato  spraying.  Similar  ar- 
rangements might  be  advantageous 
among  smaller  California  nurseries.  It 
is  possible  that  someone  could  operate 
a  profitable  business  supplying  steam  or 
renting  boilers  to  small  nurseries.  For  a 
number  of  years  in  southern  California, 
a  complete  portable  service  unit  for 
steaming  soil  in  flats  operated  success- 
fully in  a  number  of  nurseries.  A  possible 
variation  of  this  would  be  to  provide 
small  nurseries  with  uniform  soil  bins 
containing  a  pipe  grid.  When  one  was 
empty,  the  nurseryman  would  refill  it 
with  the  desired  soil  mixture.  He  might 


then  have  the  operator  steam  the  soil. 
An  additional  kind  of  work  available  to 
the  operator  of  a  portable  boiler  would 
be  the  treatment  of  outdoor  or  lath-house 
soil  beds  for  nurseries,  and  even  in  home 
yards  (Sec.  8).  Another  variation  would 
be  to  have  a  centralized  soil  service 
which  would  mix  the  specified  soil  and 
place  it  in  a  dump  truck  with  a  per- 
forated pipe  grid  (mobile  bin;  type  2) 
which  would  be  connected  to  a  large 
boiler  for  steaming.  The  soil  would  be 
tightly  covered  with  a  tarp  and  delivered, 
still  hot,  into  the  bins  of  the  nursery, 
much  as  ready-mixed  concrete  is  today. 
Variations  of  these  types  are  operating 
in  England,  and  presumably  could  do  so 
here. 

Size  of  boiler  required 
to  steam  soil 

Because  soil  steaming  may  be  done  in 
small  or  large  quantity  at  each  run, 
there  can  be  no  general  statement  of 
boiler  size  related  to  size  of  nursery. 
When  a  boiler  is  used  solely  for  steam- 
ing soil,  there  is  a  fairly  consistent  re- 
lation between  the  time  required  to  do  a 
given  job  and  the  required  size  of  boiler. 
However,  in  nurseries  using  steam  heat 
there  is  a  definite  relation  between  size 
of  boiler  and  area  to  be  steamed,  and  the 
time  required  to  do  the  job  is  more 
uniform. 

The  question  of  the  size  of  boiler  re- 
quired is  related  to  the  volume  of  steam 
already  discussed  (table  14).  Because 
the  requirements  vary  with  soil  moisture, 
temperature,  and  type,  with  the  over-all 
efficiency  of  generation  and  distribution, 
and  with  the  distance  from  the  boiler, 
general  figures  may  be  quite  misleading. 
Furthermore,  the  rating  of  boilers  in 
horsepower  may  still  be  based  largely  on 
area  of  heat  exchange  between  the  fire 
and  the  water5  rather  than  on  demon - 

5  Even  this  method  of  calculation  varies  from 
8.2  to  10  sq.  ft.  of  heat-exchange  area  per  boiler 
horsepower. 


[193] 


strated  production  of  pounds  of  steam 
per  hour.  Because  of  these  facts,  the  data 
are  quite  variable.  Newhall  (1953)  gives 
l1/*?  to  6  cubic  feet  of  soil  per  boiler 
horsepower  per  hour  as  the  range  in 
commercial  practice,  with  3  cubic  feet  as 
a  working  average.  If  one  assumes  that 
each  rated  horsepower  of  a  boiler  gives 
33.475  B.t.u.,  the  over-all  efficiency  is 
then  only  28.2  per  cent  for  the  average, 
and  56.5  per  cent  for  the  maximum  vol- 
ume of  soil  treated.  Published  figures  by 
other  workers  on  commercial  operations 
fall  within  the  above  range  of  boiler 
requirements.  From  these  data  it  is  pos- 
sible to  estimate  the  size  of  boiler  that 
will  be  required  to  treat  a  given  area  or 
volume  of  soil  in  the  permissible  time. 

The  efficiency  of  the  heat-exchange 
system  from  boiler  to  soil  is  highly 
variable.  The  boiler  efficiency  may  range 
from  40  to  90  per  cent,  and  is  the  main 
point  of  lost  energy.  There  are  further 
losses  of  heat  in  the  distribution  piping, 
these  increasing  with  distance  from  the 
boiler  and  with  decreasing  pipe  size; 
this  may  range  up  to  or  above  8  per  cent 
of  the  boiler  output.  There  may  be  large 
losses  during  injection  of  steam  into 
soil,  from  escaping  vapor,  heating  of 
structural  material  of  beds,  and  so  on; 
loss  may  range  from  9  to  56  per  cent  of 
that  introduced  into  the  soil.  Morris 
(1954)  calculated  that  an  over-all  ther- 
mal efficiency  of  41  per  cent  in  the  heat- 
transfer  system  from  coal  to  soil  was  a 
good  target  for  English  growers  effi- 
ciently using  present  equipment  and  a 
buried  perforated-pipe  grid;  this  would 
represent  about  68  per  cent  efficiency  in 
the  part  of  the  system;  following  the 
boiler. 

Table  14  presents  data  on  the  volume 
of  soil  that  can  be  heated  per  hour,  and 
the  time  required  to  heat  I  cubic  yard, 
for  several  boiler  capacities  and  thermal 
efficiencies.  The  efficiency  of  the  boiler, 
distribution  system,  and  soil-injection 
processes  may  be  approximately  calcu- 
late! (see  Appendix).  Such  data  should 


be  useful  in  analyzing  the  heat-exchange 
process  for  possible  increased  efficiency. 

Type  of  Fuel  or  Power  Used  to 
Generate  Steam 

Natural  gas  . . . 

is  the  least  expensive  fuel  in  California, 
but  it  is  not  always  available  without 
excessive  piping  cost.  It  may  be  used  for 
stationary  boilers  but  not  for  portable 
ones,  and  should  never  be  piped  into  a 
glasshouse  filled  with  plants  in  order  to 
operate  a  portable  unit.  It  is  sold  on  a 
cubic-foot  basis,  each  delivering  1,100 
B.t.u. 

Fuel  oil .. . 

is  commonly  used  for  steam  boilers  be- 
cause it  is  fairly  inexpensive.  It  requires 
a  storage  tank  and  fairly  expensive 
burners,  particularly  for  the  heavier 
cheaper  oils.  It  is  quite  readily  used  in 
portable  units.  It  is  sold  on  a  gallon 
basis,  providing  about  141,000  B.t.u. 
per  gallon  for  the  No.  3  grade. 

Kerosene . . . 

is  used  by  one  commercial  portable  unit 
for  soil  treatment  (rotating-drum  type 
with  internal  flame;  type  30),  but  is 
otherwise  not  commonly  used.  It  is  sold 
on  a  gallon  basis,  each  yielding  about 
130,000  B.t.u. 

Coal . . . 

(bituminous)  is  used  solely  for  sta- 
tionary boilers  in  this  country,  and  both 
stationary  and  portable  units  in  England 
and  Europe.  It  is  both  dirty  and  incon- 
venient to  use  and  requires  a  great  deal 
of  attention,  even  with  automatic  stokers. 
It  is  sold  on  a  ton  basis  (2,000  lb.), 
yielding  about  14,000  B.t.u.  per  pound. 

Butane . . . 

is  used  where  natural  gas  is  not  available 
and  the  boiler  is  not  equipped  for  oil.  It 
can  be  used  for  portable  units.  It  yields 
about  102,000  B.t.u.  per  gallon. 


[194] 


Propane . . . 

or  bottled  gas,  is  usually  the  most  ex- 
pensive of  the  fuels.  Uses  as  for  butane. 
It  is  sold  on  a  gallon  basis,  yielding 
91,800  B.t.u.  per  gallon. 

Electricity . . . 

is  the  most  convenient,  cleanest,  and 
most  expensive  source  of  power.  Its  use 
is  restricted  to  small  boilers  where  for 
some  reason  other  sources  of  power  can- 
not be  utilized,  and  to  self-generating 
soil-treatment  units  (types  10,  11,  14  to 
16,  24,  32,  33).  It  is  sold  on  a  kilowatt- 
hour  basis,  each  k.w.h.  yielding  3,411 
B.t.u.  Because  of  the  heavy  power  de- 
mands of  any  sizable  unit,  very  heavy 
wiring  must  be  provided,  and  with  some 
units  (for  example,  electrode  type;  type 
15)  a  transformer  may  also  be  required. 
Power  requirements  reported  for  the 
various  types  of  equipment  range  from 
1.0  to  4.0  k.w.h.  per  cubic  foot  of  soil, 
with  a  working  average  of  1.5  to  2.0 
k.w.h. 

Distribution  of  Steam 

The  objective  is  to  deliver  the  steam 
at  some  distant  point  from  the  boiler 
with  a  minimum  loss  of  heat,  pressure, 
and  rate  of  flow,  and  a  minimum  of 
condensation.  Among  the  many  factors 
that  influence  the  flow  of  steam  in  the 
pipes  are  the  following. 

Length  of  the  pipe 

Lengthening  the  pipe  sharply  in- 
creases the  rate  of  pressure  drop  and 
heat  loss.  Thus,  Morris  (1954)  calcu- 
lated the  heat  loss  for  an  uninsulated 
1%-in.  steel  pipe  carrying  25  lb.  steam 
per  min.  at  100  lb.  pressure  to  be  3.1  per 
cent  at  100  ft.,  5.8  per  cent  at  200  ft., 
and  8.3  per  cent  at  300  ft.,  the  pressure 
drop  was  21,  46,  and  95  lb.  in  the  same 
distances.  A  21/^-in.  uninsulated  steel 
pipe  with  10  lb.  pressure  and  the  same 
flow  lost  2.3  per  cent  of  its  heat  in  100 
ft.,  4.6  per  cent  in  200  ft.,  and  6.9  per 


cent  in  300  ft.;  the  pressure  drop  was 
2.8,  5.9,  and  9.3  lb.,  respectively.  By  con- 
trast, an  uninsulated  2^-in.  aluminum 
pipe  carrying  25  lb.  steam  per  min.  at 
10  lb.  pressure  lost  1.2  per  cent  of  its 
heat  in  100  ft.,  2.5  per  cent  in  200  ft., 
and  3.7  per  cent  in  300  ft. 

Diameter  of  pipe 

Increasing  the  diameter  of  the  pipe 
decreases  the  pressure  drop  and  heat 
loss;  there  may  be  a  4-  to  6-fold  increase 
in  the  rate  of  flow  by  increasing  the  pipe 
size  from  1  inch  to  2  inches.  Senner 
(1934)  found,  furthermore,  that  a  main 
used  for  soil  steaming  would  carry  ap- 
proximately 4  times  as  much  steam  as 
one  of  the  same  size  used  for  glasshouse 
heating.  This  was  due  to  the  free  flow  of 
steam,  unrestricted  by  the  back  pressure 
of  a  closed  system.  This  factor  should  be 
considered  if  the  mains  are  to  be  used 
only  for  soil  steaming,  cannot  be  if  they 
also  serve  as  heat  mains. 

Type  of  pipe 

The  kind  of  pipe  greatly  influences  the 
heat  loss,  as  the  above  figures  show. 
Morris  (1954)  has  suggested  the  use  of 
light  alloy  aluminum  irrigation  pipe  for 
steam  mains  in  low-pressure  systems, 
because  the  heat  loss  is  about  half  that 
from  steel  pipe  and  it  is  much  lighter 
in  weight.  Such  pipes,  with  light  insula- 
tion, are  also  used  in  the  Scandinavian 
countries  for  connecting  portable  boilers 
to  outdoor  beds  for  steaming. 

Steam  pressure 

The  rate  of  flow  increases  approxi- 
mately 4-fold  with  increases  in  steam 
pressure  from  10  pounds  to  100  pounds, 
with  constant  pipe  size.  The  past  practice 
of  using  high  steam  pressure  and  rela- 
tively small  pipes  is  giving  way  to  low 
pressure  and  large  pipes.  The  larger 
pipe  size  does  not  increase  heat  loss 
owing  to  the  larger  exposed  surface,  be- 
cause this  is  offset  by  the  lower  tempera- 
ture   involved.    If    insulated    aluminum 


[195] 


pipes  are  used  the  efficiency  of  steam 
distribution  will  be  greatly  improved 
over  the  old  method  (3.7  per  cent  heat 
loss  in  300  ft.  against  8.3  per  cent;  see 
above) . 

Internal  roughness  of  pipe 

The  flow  of  steam  is  decreased  by  the 
internal  roughness  of  the  pipe  and  by 
valves,  elbows,  tees,  and  reducers  in  the 
line,  since  they  increase  the  friction. 

Quality  of  steam 

Steam  quality  also  affects  the  rate  of 
flow,  decreasing  it  as  the  number  of 
water  droplets  increases.  Since  super- 
heated steam  is  relatively  drier  than 
saturated  steam,  it  flows  faster. 

Water  in  the  steam  lines 

Water  condensation  in  the  steam  lines 
should  be  prevented  for  the  above 
reasons,  because  it  represents  lost  heat, 
and  because  it  affects  the  efficiency  of 
soil  steaming.  The  condensed  water  car- 
ried into  the  soil  has  a  temperature  of 
212°  F  and,  therefore,  adds  no  heat;  the 
970  B.t.u.  per  pound  (fig.  77)  from  the 
steam  has  been  lost  in  transit.  Further- 
more, the  soil  is  made  wetter  than  it 
would    otherwise    be.    The    steam    line 


should  always  be  drained  and  bled  until 
dry  steam  appears,  before  it  is  connected 
to  the  treatment  equipment.  This  will 
prevent  the  injection  of  cold  water  into 
the  soil;  in  this  connection  it  should  be 
recalled  that  it  takes  five  times  as  many 
B.t.u.  to  heat  a  pound  of  water  as  a 
pound  of  soil.  Furthermore,  a  water  trap 
should  be  placed  in  the  line  at  a  point 
just  preceding  the  treatment  equipment, 
the  condensate  either  going  into  the  re- 
turn line  or  being  wasted.  If  there  is 
considerable  condensation  in  the  line 
this  procedure  becomes  particularly  de- 
sirable. 

Designing  the  steam 
distribution  system 

Many  of  these  factors  can  be  resolved 
for  lines  used  solely  for  soil  treatment 
by  operating  the  boiler  at  pressures  be- 
low 10  pounds,  and  using  large,  well-in- 
sulated pipes  (perhaps  of  aluminum 
alloy)  that  are  as  short  as  possible. 
However,  if  the  pipe  is  too  large,  the 
heat  loss  from  the  increased  surface 
exceeds  that  saved  by  reduced  friction. 
The  losses  may  also  be  reduced  by  using 
a  portable  boiler  close  to  the  soil  being 
treated.  A  heating  engineer  should  be 
consulted  for  the  specific  design. 


SOIL  TREATMENT  IN  A  MECHANIZED  NURSERY 


Mechanization  of  every  practicable 
nursery  procedure  is  of  increasing  in- 
terest to  the  industry,  due  to  rising  wages 
for  labor  and  smaller  margins  of  profit. 
Any  successful  mechanized  nursery  pro- 
gram must  include,  indeed  must  be  built 
around,  soil  treatment,  for  reasons  out- 
lined in  sections  2  and  3.  A  number  of 
methods  and  pieces  of  equipment  pre- 
viously described  for  soil  treatment 
integrate  very  well  into  mechanized  nur- 


sery practice,  and  13  of  these  are  shown 
schematically  in  figure  126.  By  following 
the  arrows  in  that  figure,  the  routing  of  a 
particular  procedure  may  be  visualized. 
It  should  be  emphasized  that  untreated 
flats,  cans,  or  pots  may  be  used  in  the 
procedure  prior  to  soil  treatment,  but 
that  treated  soil  should  not  be  dumped 
into  untreated  containers  or  on  an  un- 
treated floor  (Sec.  12).  Mechanization  is 
further  discussed  in  Section  17. 


[196] 


SECTION 


Chemical  Treatment 
of  Nursery  Soils 


Donald  E.  Munnecke 


Fungicides 

Nematocides 

Insecticides 

Soil  drenches  around  living  plants 


s 


OIL  may  BE  treated  with  chemicals 
to  rid  it  of  fungi,  bacteria,  insects,  nema- 
todes, and  weeds.  The  effectiveness  of 
chemical  soil  treatment  generally  de- 
creases as  the  size  of  the  treated  area 
increases.  Although  early  trials  of  soil 
fumigation  in  the  field  attempted  to 
eradicate  pathogens  and  insects,  experi- 
ence has  shown  that  this  is  impractical  if 
not  impossible;  a  few  chemicals  are, 
however,  effective  in  reducing  field  in- 
festations, and  some  of  them  are  men- 
tioned briefly  in  this  section.  On  the 
other  hand,  pathogens  can  be  eradicated 
from  soil  in  containers  such  as  flats, 
where  the  chemicals  do  not  need  to 
penetrate  large  soil  masses.  Steam  treat- 
ment is  usually  more  satisfactory,  but 
there  are  many  occasions  where  chemi- 
cals can  be  used  more  cheaply  and 
effectively  and  their  use  has  increased 
tremendously. 

The  cost  of  chemical  fungicide  treat- 
ment using  methyl  bromide  or  chloro- 
picrin  varies  from  approximately  1  to  3 
cents  per  cubic  foot,  according  to  the 
method  and  chemicals  used  (table  16). 
The  present  practice  is  to  use  the  chemi- 


cal and  dosage  which  will  give  the 
greatest  net  return  to  the  grower,  as 
determined  by  practical  experience.  It 
is  difficult  to  recommend  single  dosages 
of  chemicals  for  all  soils  and  all  condi- 
tions. In  general,  the  lower  dosage 
recommended  herein  is  for  use  with  a 
U.  C.-type  mix;  the  higher  dosages  are 
recommended  for  use  on  clay  soils  or 
soils  containing  undecayed  organic 
matter. 

An  ideal  chemical  for  treating  soil  is 
one  that  kills  a  variety  of  fungi,  bacteria, 
insects,  and  weeds;  is  inexpensive  and 
harmless  to  the  operator  and  equipment ; 
is  quick-acting  and  effective  deep  in  the 
soil  as  well  as  on  the  surface ;  is  harmless 
to  near-by  plants;  and  is  nontoxic  to 
subsequent  plantings  in  the  soil.  None  of 
the    presently    known    chemicals    fulfills 


CAUTION:  Many  of  the  chemicals 
mentioned  in  this  manual  are  poi- 
sonous and  may  be  harmful.  The 
user  should  carefully  follow  the  pre- 
cautions on  the  labels  of  the  con- 
tainers. 


[197] 


Table  1 6.  Comparison  of  the  Cost  in  1 955  of  Methyl  Bromide  and 

Chloropicrin  Used  to  Treat  Soil  for  Fungus  Control 

Exclusive  of  labor  and  cost  of  accessory  equipment 


Cost 

Dosage 

Flats* 

Bulk 

Surface 

Chemical 

Cents  per 
flat 

Cents  per 
cu.  ft. 

Cents  per 
sq.  ft. 

$1.65  per  lb. 

3  cc 

1.8 

1.8 

(1-lb.  lot) 

5  cc 

3.0 

Chloropicrin 

$1.25  per  lb. 

3  cc 

1.4 

1.4 

(25-lb.  lot) 

5  cc 

2.3 

$1.02  per  lb. 

3  cc 

1.1 

1.1 

(100-lb.  lot) 

5  cc 

1.9 

$0.80  per  lb. 

4  lb.  per 

(1-lb.  lot) 

100  cu.  ft. 

2.2 

3.2 

3.2 

Methyl  bromide 

$0.72  per  lb. 

4  lb.  per 

(50-lb.  lot) 

100  cu.  ft. 

1.9 

2.9 

2.9 

*  Chloropicrin  not  recommended  for  flats.  Flats  18  X  18  X  3  inches.  These  figures  based  upon  40  flats 
per  cu.  yd.  of  air  space.  In  actual  practice  the  cost  varies  with  the  way  the  flats  are  stacked  and  the  dosage 
used.  The  dosages  given  are  sufficient  to  eradicate  pathogens  in  confined  areas.  The  lower  dosage  of  chloro- 
picrin is  sufficient  for  this  purpose  with  a  U.C.-type  mix;  the  higher  dosage  may  be  necessary  for  clay  soils 
or  those  high  in  manure  or  other  undecayed  organic  matter. 


all  of  these  requirements,  but  many  ful- 
fill enough  of  them  for  practical  use. 
Much  research  is  being  done  on  this 
problem  and  it  is  probable  that  some 
materials  will  be  available  in  the  future 
that  approximate  this  ideal.  The  most 
common  chemicals,  dosages,  and  treat- 
ments used  to  control  diseases  and  nema- 
todes in  nursery  soils  are  summarized  in 
table  17  at  the  end  of  this  section. 

Special  chemicals  and  dosages 
required  to  kill  fungi 

Fungi,  such  as  Rhizoctonia,  Fusarium, 
Armillaria,  and  Verticillium  are,  in  gen- 
eral, more  difficult  to  kill  in  the  soil  with 
chemicals  than  are  insects,  nematodes, 
and  most  weed  seeds.  This  fact  must  be 
kept  clearly  in  mind  when  a  chemical  is 
chosen  for  soil  treatment.  For  example, 
it  is  a  waste  of  time  and  money  to  use 
ethylene  dibromide,  an  excellent  noma- 
tocide,    to    try    to    control    damping-off, 


which  is  caused  by  fungi.  Also,  low 
dosages  of  methyl  bromide,  an  excellent 
soil  fungicide,  may  be  used  for  weed 
and  nematode  control,  but  two  to  four 
times  as  much  may  be  required  for  fun- 
gus control.  Growers  often  erroneously 
believe  that  if  weeds  and  nematodes  are 
controlled,  the  fungi  are  also.  If  fungi 
are  controlled,  however,  insects  and 
nematodes  are  usually  eliminated.  //  the 
main  problem  is  fungus  control,  suitable 
fungicides  at  recommended  dosages  must 
be  used.  Nematocides  or  herbicides  can- 
not be  used  to  control  fungi. 

Soil  preparation,  temperature, 
and  aeration 

The  condition  and  temperature  of  the 
soil  must  be  considered  in  using  chemi- 
cals for  soil  treatments.  A  good  rule  to 
follow  is  that  the  soil  should  be  in  good 
planting  condition  before  treatment.  The 
soil  should  be  in  good  tilth,  and  there 


[198] 


should  be  no  lumps  or  clods.  Too  much 
soil  water  prevents  thorough  diffusion 
of  the  gas  (Sec.  9),  whereas  too  little 
moisture  on  the  soil  surface  allows  the 
gas  to  escape.  Best  results  are  obtained  at 
soil  temperatures  of  65°  to  75°  F.  Where 
soil  is  likely  to  be  cold  and  wet  for  long 
periods,  chemical  treatment  may  be 
facilitated  by  storing  it  in  bins  in  a 
heated  shed.  It  may  then  be  treated  in 
place. 

After  treatment  the  soil  must  be 
thoroughly  aerated  so  that  all  trace  of 
the  fumigant  is  gone  before  planting. 
In  general,  the  higher  the  temperature 
and  the  lighter  the  soil,  the  shorter  the 
aeration  period.  In  all  cases  the  chemi- 
cals should  be  handled  with  care,  as 
should  any  poisonous  substance. 

After-effects  of  treatment 

In  some  cases  an  increased  growth 
response  is  apparent  in  chemically 
treated  soils  (fig.  119).  This  is  not  be- 
cause the  chemicals  act  as  fertilizers,  but 
probably  because  the  soil-borne  patho- 
gens and  pests  are  eliminated  and  a  more 
favorable  balance  of  the  other  micro- 
organisms is  obtained  (Sec.  14). 

Occasionally  soil  treated  with  chemi- 
cals is  toxic  to  subsequent  plantings.  This 
toxicity  is  usually  due  to  insufficient 
aeration  of  the  soil  after  treatment.  Soils 
which  are  high  in  some  organic  ma- 
terials or  clay,  excessively  wet,  or  treated 
at  low  temperatures  may  contain  toxic 


amounts  of  the  chemicals  several  weeks 
after  application.  These  ill  effects  are  due 
to  a  residue  of  the  gas,  to  a  breakdown 
product  of  it,  or  to  secondary  reactions 
of  soil  microorganisms  causing  such  re- 
sults as  ammonium  accumulation.  Usu- 
ally this  toxicity  may  be  avoided  by 
applying  the  chemicals  correctly  and  by 
delaying  planting  until  all  odor  of  the 
chemicals  is  gone.  The  use  of  a  U.  C.-type 
soil  mix  greatly  reduces  the  chance  of 
injury  from  chemical  treatments,  another 
reason  for  adopting  such  a  system. 

Recently  the  chemicals  containing 
bromine  (methyl  bromide,  ethylene  di- 
bromide,  chloro-bromo-propene)  have 
been  found  to  leave  a  residue  which  is 
extremely  toxic  to  certain  plants  (espe- 
cially carnations)  ;  these  compounds 
should  not  be  used  on  soil  to  be  planted 
to  carnations.  Although  a  large  number 
of  bedding  plants  and  other  crops  have 
been  planted  in  chemically  treated  soil 
without  harm,  it  is  a  good  precaution  to 
use  chemicals  on  a  small  scale  on  an  un- 
tried crop  and  note  the  results  before 
treating  large  quantities  of  soil. 

Formaldehyde  treatment 
of  floor 

Whenever  treated  soil  is  dumped  in 
bulk  piles  on  the  floor,  the  surface  should 
previously  have  been  wet  down  with  a 
formaldehyde  solution  (1  gal.  to  18  gal. 
of  water) . 


FUNGICIDES 


A  wide  variety  of  chemicals  is  mar- 
keted for  soil  treatment  of  one,  sort  or 
another.  The  various  chemicals  are  here 
grouped  according  to  the  purpose  to 
which  they  are  suited.  First  let  us  con- 
sider fungicides. 

The  soil  troubles  caused  by  fungi  are, 
in  general,  the  most  difficult  to  control 
with  chemicals.  Some  fungi  are  able  to 
survive  in  soil  for  many  years  in  the  ab- 


sence of  their  host  plants.  Most  of  them 
form  thick-walled  resting  bodies  which 
are  resistant  both  to  unfavorable  en- 
vironmental conditions  and  to  chemical 
treatments  (Sec.  3) .  A  number  are  capa- 
ble of  growing  or  persisting  deep  in  the 
soil,  well  below  the  depths  reached  by 
the  surface  treatments  of  the  soil.  The 
most  effective  soil  fungicides  are  fumi- 
gants  which  act  as  gases  in  the  soil. 


[199] 


Chloropicrin,  methyl  bromide,  and 
formaldehyde  are  the  most  widely  used 
soil  fungicides.  These  compounds  are 
liquids  whose  gaseous  phases  diffuse 
through  soil.  When  properly  applied  in 
adequate  dosages,  they  control  most  of 
the  fungi,  nematodes,  and  weeds.  The 
gases  have  to  be  confined  in  some  way 
during  the  treatment  period.  After  treat- 
ment the  soil  must  be  thoroughly  aerated 
before  planting. 

For  sources  of  soil-treatment  materials 
in  California  see  the  Appendix.  It  is  not 
necessary  to  get  a  license  to  use  these 
chemicals. 

Chloropicrin 

Chloropicrin,  or  tear  gas,  is  extremely 
toxic  to  soil  fungi,  insects,  weed  seeds, 
and  nematodes.  It  penetrates  bulk  soil 
readily,  but  does  not  readily  penetrate 
plant  tissue  such  as  unrotted  nematode 
galls.  It  is  neither  explosive  nor  in- 
flammable, but  it  is  difficult  to  use  since 
it  is  a  potent  tear  gas. 


To  avoid  eye  irritation,  start  working 
on  the  windward  side  of  the  planting 
with  the  back  to  the  wind  so  that  the 
fumes  are  carried  away  from  the  eyes. 
Do  not  rub  eyes  when  affected;  rather, 
turn  and  face  the  wind  and  let  the  effects 
wear  off.  Since  it  is  very  corrosive  to 
most  metals  (but  not  to  stainless  steel), 
metal  equipment  must  be  washed  with 
kerosene  after  using.  It  cannot  be  used 
in  the  vicinity  of  living  plants;  conse- 
quently, all  plants  in  a  glasshouse  must 
be  removed  before  treatment.  It  may  be 
used  in  semienclosed  areas,  such  as  lath 
houses,  provided  there  is  ample  air 
movement  to  dissipate  the  gas,  and  ad- 
jacent plants  are  over  3  feet  away  from 
the  treatment  area. 

Heavy  soil  absorbs  large  quantities  of 
the  gas  and  requires  larger  dosages  and 
longer  aeration  periods  than  light  soil. 

Chloropicrin  may  be  applied  by  ma- 
chines in  field  applications,  or  by  hand- 
operated  injectors    (fig.  105).  For  sur- 


T 


K> 


1 


Fig.  105.  Hand-injector   for   applying    soil   fumigants,   at   left.    Proper   spacing   shown   at   right 
for  hand-injection  of  fumigants  into  soil  in  beds,  benches,  or  fields. 


[200] 


face  applications  the  dosage  is  3  cc  per 
12-inch  square,  injected  6  inches  deep; 
for  bulk  soils  the  dosage  is  3  to  5  cc  per 
cubic  foot.  Bulk  soils  may  be  treated  in 
bins,  drums,  garbage  cans,  or  any  gas- 
proof receptacle  that  can  be  tightly 
sealed.  The  gas  may  be  confined  in  fields 
by  wetting  the  top  inch  of  soil  or  by  en- 
closing beneath  a  gasproof  cover  (poly- 
ethylene types,  such  as  Visqueen,  are 
most  common)  for  1  to  3  days.  In  gen- 
eral, plantings  must  be  delayed  7  to  10 
days  after  treating  or  until  all  traces  of 
the  gas  have  disappeared.  Do  not  use 
if  soil  temperatures  at  6-inch  depth  are 
below  60°  F;  best  results  are  obtained 
at  70°. 

Chloropicrin  is  specifically  recom- 
mended in  soil  used  for  chrysanthemums 
(dosage:  3  cc  on  12-inch  centers,  6 
inches  deep)  and  carnation  plantings  in 
California,  since  methyl  bromide  is 
sometimes  unsatisfactory  for  these  crops. 

With  proper  application  and  aeration, 
chloropicrin-treated  soil  gives  excellent 
disease  control  and  plant  growth,  and  it 
is  generally  considered  as  the  experi- 
mental standard  when  comparing  soil 
fungicides.  However,  it  is  difficult  to 
handle,  it  requires  much  labor  to  apply, 
and  the  treatment  periods  are  long. 
Aeration  of  the  soil  after  treatment  is 
essential  since  a  number  of  crops  have 
been  lost  through  faulty  aeration.  Con- 
sequently, it  is  not  recommended  for 
flat-soil  operations  nor  for  other  circum- 
stances where  soil  must  be  used  soon 
after  treating.  See  table  16  for  approxi- 
mate cost  of  chloropicrin. 

Methyl  Bromide 

Methyl  bromide  is  widely  used,  espe- 
cially for  flat-  and  bulk-soil  sterilization. 
It  has  been  approved  by  the  California 
Department  of  Agriculture  for  treatment 
of  soil  in  which  certain  plants  may  be 
grown  if  they  are  to  be  shipped,  using 
the  Intercounty  Nursery  Stock  Certifi- 
cate or  "pinto"  tag  (sees.  3  and  8).  It  is 
effective  against  most  of  the  soil  pests, 


is  simple  to  use,  and  has  the  shortest 
treatment  and  aeration  period  of  the 
present  soil  fungicides. 

Although  it  is  extremely  toxic  to  man, 
there  is  little  danger  if  it  is  handled  with 
reasonable  care.  Take  adequate  precau- 
tion to  prevent  exposure  of  children  and 
other  persons  to  the  poisonous  gas.  Post 
warning  signs. 

It  is  marketed  in  several  forms  (pure 
methyl  bromide,  methyl  bromide  with  2 
per  cent  chloropicrin,  or  as  a  liquid  in 
various  solvents),  but  the  pure  gas  is 
almost  exclusively  used  in  California  and 
the  following  recommendations  apply 
to  this  gaseous  form. 

Field  soils,  ground  beds,  flats,  pots, 
cans,  tools,  and  even  large  trucks  and 
farm  machinery  may  be  satisfactorily 
treated  with  methyl  bromide  by  enclos- 
ing them  beneath  a  gasproof  cover 
(polyethylene  is  cheapest;  polyvinyl 
chloride  and  vinyl-coated  nylon  are 
more  durable,  but  more  expensive)  and 
injecting  the  gas  beneath  the  cover 
through  special  applicators  from  cans 
or  cylinders  (figs.  106  and  107).  The 
cover  must  be  examined  carefully  for 
holes;  these  may  be  sealed  with  masking 
tape.  The  cover  must  be  sealed  tightly 
with  soil  around  the  edges  before  apply- 
ing the  gas.  Never  use  the  gas  at  soil 
temperatures  below  50°  F,  preferably 
around  70°.  For  treatment  of  contain- 
ers see  Section  12. 

Treating  stacks  of  flats 

Up  to  400  flats  can  be  adequately 
treated  in  one  stack  as  follows. 

Fill  the  flats  with  soil  moistened  in 
preparation  for  planting,  but  not  pud- 
dled or  rendered  soggy.  Place  them  on 
a  level  surface  or  hard  dirt  area  and 
stagger  them  as  shown  in  figure  107,  or 
use  lath  spacers  between  flats  to  allow 
free  circulation  of  the  gas.  Fasten  the 
outlet  of  a  rubber  or  plastic  hose  to  the 
top  of  the  center  flat  so  that  it  points 
away  from  the  soil.  Place  burlap  or  rags 
over  the  corners  of  the  flats  to  prevent 


[201] 


damage  to  the  cover.  Put  the  cover 
loosely  over  the  top  and  seal  the  edges 
with  a  dirt  seal  or  with  "sand  snakes" 
(canvas  or  plastic  tubes  3  inches  in  di- 
ameter filled  with  sand). 

The  gas  may  be  more  effectively  ap- 
plied by  heating  it  in  one  of  several  ways 
before  injecting  beneath  the  cover.  The 
simplest  way  is  to  immerse  the  cans  in 
a  bucket  of  water  heated  to  140°  to 
160°  F,  or  to  route  the  gas  through  a 
copper  coil  immersed  in  hot  water  (fig. 
106) .  If  large  cylinders  are  used,  the  gas 
may  be  circulated  through  heated  coils 
in  the  same  manner.  A  supplementary 
heating  device  may  be  used  to  keep  the 
water  hot. 

Avoid  turning  cans  upside  down  while 
injecting.  If  cans  are  turned  upside 
down,  liquid  methyl  bromide  will  flow 
through  the  hose.  This  reduces  effective- 
ness and  may  cause  the  soil  to  be  toxic 
near  the  hose  exit. 

To  calculate  the  amount  of  chemical 


to  use,  multiply  the  length,  breadth,  and 
height  of  the  stack  in  feet,  including  the 
air  space  beneath  the  cover.  Use  4 
pounds  of  methyl  bromide  per  100  cubic 
feet  for  fungus  control  (damping-off, 
root  rot,  etc.),  or  1%  to  2  pounds  per 
100  cubic  feet  for  weed  and  nematode 
control.  A  special  metering  device  (fig. 
106)  is  advisable  for  use  on  cylinders, 
or  a  scale  may  be  used  to  actually  weigh 
the  gas  as  it  is  being  dispensed.  Place  the 
cylinder  on  a  leveled  scale,  note  its 
weight,  open  the  valve  and  allow  the  gas 
to  flow  until  the  scale  indicates  that  the 
desired  amount  of  gas  has  been  used. 
Close  the  valve  and  recheck  the  weight. 
The  initial  weight  minus  the  final  weight 
equals  the  weight  of  methyl  bromide  ap- 
plied. 

After  a  24-  to  48-hour  treatment  pe- 
riod, remove  the  cover  and  allow  the 
flats  to  aerate  for  24  to  48  hours,  when 
they  are  ready  for  planting. 


Fig.  106.  Equipment  for  applying  gaseous  methyl  bromide  to  nursery  soils.  Left,  gas  metered 
from  a  cylinder,  for  large  operations.  Right,  applicator  for  1-lb.  cans  of  gas  for  small  operations. 


T  202  1 


Fig.  107.  Diagrams  showing  the  methods  for  stacking,  covering,   sealing,  and   injecting   flats 
of  soil  with  gaseous  methyl  bromide. 


Treating  bulk  soil 
and  ground  beds 

Bulk  soil  and  ground  beds  may  be 
treated  in  the  same  way.  Pile  bulk  soil 
1  to  2  feet  deep,  be  sure  that  it  is  not 
excessively  wet  or  cold,  and  treat  as 
shown  in  figure  107.  For  ground  beds, 
measure  the  area  beneath  the  cover  and 
apply  at  4  pounds  per  100  square  feet  for 
fungus  control,  or  1%  to  2  pounds  per 
100  square  feet  for  nematode  and  weed 
control  (fig.  125). 

Effectiveness 

Methyl  bromide  kills  nematodes  even 
in  unrotted  galls,  but  it  does  not  kill 
V erticillium  albo-atrum  (which  causes 
wilt  of  chrysanthemum  and  numerous 
other  crops)  and  it  may  leave  the  soil 
toxic  for  carnations.  Consequently,  it  is 
not  recommended  for  carnations  or 
chrysanthemums.  It  is  recommended  for 
bedding-plant  operations  where  a  rapid 
turnover    of   soil   and   flats   is   required 


and  the  short  treatment  period  is  neces- 
sary. 

The  recommended  dosage  of  4  pounds 
per  100  cubic  feet  will  control  all  of  the 
common  soil  fungus  diseases  except 
V erticillium  wilt.  Excellent  weed  con- 
trol is  obtained  except  for  Malva  and  bur 
clover.  The  user  should  not  think  treat- 
ment unsuccessful  if  these  weeds  sur- 
vive, but  if  many  other  weeds  survive, 
it  indicates  that  the  application  was 
faulty.  Lower  dosages  have  been  suc- 
cessfully used  in  warm  sandy  soils,  and 
growers  have  found  that  dosages  as  low 
as  2  pounds  per  100  cubic  feet  may  be 
effective. 

Table  16  shows  the  approximate  cost 
of  methyl  bromide  based  upon  the  maxi- 
mum dosage. 

Formaldehyde 

Formaldehyde  is  the  chemical  that  has 
been  in  longest  use  for  soil  fumigation, 
and  is  still  used  as  a  drench  for  rooting 


[203] 


media  and  glasshouse  benches.  It  is  a 
water-soluble  liquid  that  penetrates 
the  soil  as  far  as  the  water  carrier  and 
volatilizes  rapidly,  one  of  the  principal 
advantages  of  this  material.  Since  the 
fumes  are  very  toxic  to  near-by  foliage, 
it  has  restricted  use  in  glasshouses  with 
living  plants.  It  has  been  successfully 
used  in  large  glasshouses  when  the  vents 
were  fully  open  and  the  treatment  area 
was  somewhat  removed  from  living 
plants.  Formaldehyde  is  very  irritating 
to  eyes  and  nasal  passages,  and  is  ob- 
noxious to  use  for  this  reason. 

Commercial  formaldehyde  (37  to  40 
per  cent  formaldehyde  in  water  solu- 
tion) at  1  pint  in  61/4  gallons  of  water 
is  applied  at  the  rate  of  %  gallon  per 
square  foot  of  soil  surface.  Soil  is  cov- 
ered for  24  hours  with  plastic  covers  or 
gas-resistant  paper  and  then  it  is  aerated 
by  thorough  stirring. 

The  aeration  period  is  from  10  to  14 
days.  The  residual  effects  are  very  dam- 
aging to  seedlings  and  transplants  and 
care  must  be  taken  that  all  odor  of  the 
gas  is  gone.  A  sure  way  to  determine 
whether  soil  is  safely  aerated  is  to  plant 
a  few  seedlings  in  it  and  see  whether  in- 
jury results. 

A  dilute  method  has  been  prescribed 
for  damping-off  control  which  may  be 
effective  with  lightly  infested  soils.  It  is 
ineffective  in  heavily  infested  soil.  By 
this  method  2  tablespoons  (1  fl.  oz.)  of 
formaldehyde  in  %  cup  (6  fl.  oz.)  of 
water  are  sprinkled  over  a  cubic  foot  of 
moist  soil  which  has  been  spread  in  a 
thin  layer;  the  soil  is  then  mixed  thor- 
oughly, and  stacked  in  clean  flats  and 
covered  with  a  tarpaulin  or  wet  paper. 
After  24  hours  seeds  may  be  sown,  pro- 
vided the  flats  are  watered  thoroughly 
afterwards. 

Vapam 

Vapam,  sodium  N-methyl  dithio- 
carbamate  dihydrate,  has  recently  been 
released  for  commercial  use  as  a  general 
soil   fumigant.   It  decomposes  in  soil  to 


form  methyl  isothiocyanate  gas,  which 
is  the  active  killing  agent.  It  has  been 
successfully  used  to  control  weeds,  nema- 
todes, soil  insects,  and  soil  fungi,  al- 
though it  is  not  recommended  for  eradi- 
cative  treatments.  Since  Vapam  is  solu- 
ble in  water,  it  may  be  applied  by  sprin- 
kling or  irrigating  on  the  surface  of  the 
soil  or  by  injecting  into  soil  with  stand- 
ard equipment.  Consequently  it  is  one 
of  the  most  versatile  of  the  soil  fumi- 
gants.  It  has  sufficient  advantages  to 
warrant  use,  especially  on  crops  grown 
in  the  field  or  in  ground  beds.  Although 
Vapam  may  be  used  for  treating  flat  or 
potting  soil,  methyl  bromide  is  usually 
more  satisfactory  for  these  purposes. 

Vapam  is  sold  as  a  liquid  containing 
4  pounds  of  the  active  material  per  gal- 
lon of  water.  Dosages  are  usually  ex- 
pressed as  quarts  or  gallons  of  this  for- 
mulation to  be  applied  to  a  given  area 
of  soil.  The  dosage  required  depends 
upon  the  organisms  which  are  to  be  con- 
trolled; for  example,  for  weed  control 
it  is  from  1  to  l1/^  pints  per  100  square 
feet  and  for  fungus  control  it  is  from  1 
to  2  quarts  per  100  square  feet. 

Treating  beds  or  benches 

For  small  areas,  such  as  ground  beds 
or  benches,  1  quart  of  Vapam  in  2  to  3 
gallons  of  water  in  a  sprinkling  can  may 
be  applied  uniformly  over  the  surface  of 
100  square  feet  of  soil  and  immediately 
followed  with  enough  water  to  wet  the 
soil  to  a  depth  of  6  inches.  This  dosage 
is  approximately  equivalent  to  100  gal- 
lons per  acre,  which  is  a  good  median 
application  for  fungus  control. 

A  hose  proportioner  may  be  used  in 
the  same  way  with  a  stock  solution  of 
1  quart  of  Vapam  in  1  quart  of  water. 
The  proportioner  ratio  should  not  exceed 
1:20. 

Treating  fields 

For  field  applications  either  overhead 

sprinklers  or  soil  injectors  may  be  used. 

Overhead   sprinkler  applications  may 


[204] 


be  made  by  introducing  Vapam  into  the 
lines.  The  field  is  sprinkled  for  5  to  10 
minutes,  then  the  required  amount  of 
Vapam  is  introduced  in  the  next  10  to  20 
minutes.  The  sprinklers  are  then  left  on 
until  the  soil  is  wet  to  a  depth  of  6  to  12 
inches. 

Vapam  may  be  applied  with  stand- 
ard knife-blade  soil  injectors  set  6  inches 
apart  and  4  inches  deep.  The  injection 
method  is  useful  because  rows  or  strips 
may  be  easily  and  inexpensively  treated. 
The  row-treatment  method  has  been 
used  with  many  field  crops  such  as  cot- 
ton and  beans,  but  surface  applications 
Lave  been  more  widely  used  with  orna- 
mental crops. 

As  with  all  effective  chemical  soil 
treatments,  the  soil  should  be  in  seedbed 
condition  at  the  time  of  treatment.  Opti- 
mum soil  temperatures  at  the  4-inch 
depth  are  from  55°  to  65°  F,  although 
Vapam  has  been  successfully  used  at 
temperatures  of  45°  to  70°.  The  tem- 
peratures of  California  soils  at  planting 
time  do  not  ordinarily  restrict  its  use, 
although  high  temperatures  may  vola- 
tilize the  gas  too  rapidly,  and  surface  ap- 
plications should  not  be  made  on  hot 
days. 

Seven  days  after  treating,  or  when 
soil  has  dried  to  a  workable  condition, 
the  soil  should  be  cultivated  lightly  to 
break  the  crust  and  facilitate  aeration. 
Usually  treated  soil  may  be  planted  14 
days  after  treatment  unless  prolonged 
rains  have  prevented  adequate  aeration. 
In  such  cases  it  is  desirable  to  wait  until 
all  odor  of  the  gas  is  gone,  or  to  make 
test  plantings  before  planting  the  main 
crop. 

Terraclor 

Terraclor  (PCNB),  pentachloronitro- 
benzene,  has  recently  been  marketed 
as  a  soil  fungicide.  It  differs  from 
most  soil  fungicides  in  its  specificity  of 
action  (for  example,  Rhizoctonia  is  con- 
trolled, Pythium  is  not),  its  low  toxicity 


to  many  plants,  and  its  relatively  long 
residual  activity  in  soil.  Another  peculi- 
arity of  the  compound  is  that  it  inhibits 
growth  of  Rhizoctonia,  but  does  not  kill 
it.  Because  of  these  factors  it  may  be 
used  in  several  ways  for  disease  control, 
provided  water  molds  are  not  present. 
The  compound  is  new  and  it  must  be 
tried  on  many  more  ornamental  plants 
before  it  can  be  generally  recommended. 
The  results  on  carnations  obtained  by 
Sciaroni  and  Raabe  (1955)  and  Scia- 
roni  (1955)  are  cited  here  as  a  guide  for 
use  on  other  crops  as  well  as  for  carna- 
tions. 

As  a  preplanting  treatment  the  75  per 
cent  wettable  powder  PCNB  may  be 
dusted  or  sprayed  on  the  soil  surface  at 
the  rate  of  1  to  1%  pounds  per  1,000 
square  feet  and  mixed  to  a  depth  of  1  to 
2  inches  by  raking.  Carnation  plants 
may  be  set  immediately  in  soil  treated 
in  this  manner  without  damage.  Damp- 
ing-off  caused  by  Rhizoctonia  has  also 
been  effectively  controlled  in  sweet-basil 
plantings  by  application  of  the  75  per 
cent  wettable  powder  at  1  to  1%  pounds 
per  1,000  square  feet  immediately  before 
seeding.  No  seedling  injury  has  been  ob- 
served. 

In  addition  to  the  soil-surface  method 
of  application,  PCNB  has  been  applied 
as  a  strip  treatment  in  the  planting  fur- 
row at  the  time  of  planting.  A  general 
recommendation  for  this  method  with 
beans  and  cotton  is  to  use  5  pounds  of 
the  75  per  cent  material  in  10  gallons  of 
water  per  acre. 

A  unique  use  of  PCNB  has  been  its 
application  as  a  protective  fungicide  on 
soil  previously  treated  with  steam  or 
chemicals  such  as  chloropicrin,  Vapam, 
or  methyl  bromide.  To  protect  against 
infestation  (sees.  3  and  14)  of  such 
treated  soil  by  Rhizoctonia,  1  to  1% 
pounds  PCNB  (75  per  cent  wettable 
powder)  is  applied  to  1,000  square  feet 
of  soil  and  raked  into  the  top  1  to  2 
inches  just  prior  to  planting. 


[205] 


NEMATOCIDES 


Chloropicrin  and  methyl  bromide  eco- 
nomically control  nematodes  in  con- 
tainer soils  when  other  pathogens  must 
also  be  killed.  For  controlling  nema- 
todes alone  in  field  soils,  however,  these 
materials  are  too  expensive.  If  nematodes 
or  soil  insects  are  the  problem  one  of 
these  nematocides  may  be  used;  but  it 
should  not  be  used  for  control  of  damp- 
ing-off  or  other  fungus  diseases.  For 
these  reasons  the  recommendations 
given  for  nematocides  deal  with  field  ap- 
plications only.  The  same  general  soil 
preparation  as  outlined  for  fungicides  is 
required  for  successful  treatment  for 
nematodes. 

Ethylene  Dibromide 

Ethylene  dibromide  is  a  liquid  which 
is  applied  to  soil  by  a  hand-injector  or 
by  a  continuous-flow  applicator  at  the 
rate  of  3V2  to  7  gallons  of  actual  ethylene 
dibromide  per  acre.  Since  it  is  not  so 
volatile  as  chloropicrin  or  methyl  bro- 
mide, special  covers  or  seals  are  not  re- 
quired for  its  use.  This  relatively  low 
volatility  necessitates  long  aeration  pe- 
riods of  2  to  3  weeks  before  planting.  It 
should  not  be  used  on  soil  to  be  planted 
to  carnations. 

D-D  Mixture 

D-D  mixture  (dichloropropane-dichlo- 
ropropene)    was  the  first  of  the  cheap 


effective  nematocides.  It  is  applied  by 
hand-injection  or  continuous-flow  appli- 
cators at  the  rate  of  200  to  400  pounds 
per  acre.  Planting  must  be  delayed  1  to 
2  weeks. 

Nemagon  and  V-C  13 

Nemagon  (l,2-dibromo-3-chloropro- 
pane)  and  V-C  13  (0-2,4-dichlorophenyl 
0,  0-diethyl  phosphorothioate)  are  newly 
marketed  nematocides  which  are  said  to 
be  nontoxic  to  many  plants  when  applied 
to  soil  around  established  root  systems. 
They  represent  promising  developments 
in  nematode  control,  but  cannot  yet  be 
recommended  for  general  use. 

General  Effectiveness  of 
Nematocides 

These  chemicals  are  very  cheap  and, 
although  nematodes  are  not  completely 
eradicated  by  treatment,  sufficient  con- 
trol is  obtainable  to  return  the  cost  of 
the  treatment  many  times. 

Many  field-flower  growers  are  now 
using  metering  valves  mounted  above 
the  plowshares  to  drip  soil  fumigants 
into  the  furrow  during  the  plowing  op- 
eration. This  practice  fits  in  well  with 
the  culture  of  most  crops  and  has  re- 
duced the  cost  of  nematode  control, 
since  expensive  injection  machines  are 
not  necessary. 


INSECTICIDES 

The  common  soil  insects  are  all  killed  problem,  two  recently  published  pest- 
by  treatments  which  are  effective  against  control  guides  (Jefferson  and  Pritchard, 
fungi  and  nematodes,  hence  the  specific  1956;  Pritchard,  1949)  may  be  con- 
soil  insecticides  are  not  discussed  here,  suited  for  a  description  of  the  insecti- 
In   the  event  that  insects  are  the  main  cides  and  treatments. 


[206] 


SOIL  DRENCHES  AROUND  LIVING  PLANTS 


A  few  fungicides  have  been  used  as 
soil  drenches.  These  are  applied  to  the 
soil  around  the  growing  plant  and  are 
used  primarily  to  prevent  enlargement 
of  an  existing  infestation.  They  are  ma- 
terials of  limited  volatility  which  kill  by 
direct  contact  with  the  parasites.  They 
are  not  recommended  as  primary  dis- 
ease-control materials,  but  they  may  be 
useful  in  checking  the  spread  of  disease 
that  arises  in  a  planting  in  spite  of  previ- 
ous precautions.  Soil  drenches  of  this 
type  must  penetrate  the  soil,  kill  or  in- 
hibit the  fungus  parasites,  and  be  non- 
toxic to  the  existing  plants.  Drenches 
may  suppress  pathogens  but  do  not  eradi- 
cate them,  since  none  of  the  present  ma- 
terials are  able  to  penetrate  soils  more 
than  a  few  inches.  In  addition,  many  of 
them  are  rapidly  inactivated  in  soil. 

The  most  commonly  used  materials 
for  soil  drenches  are  ferbam  (Fermate), 
Semesan,  thiram  (Arasan),  captan 
(Ortho  406),  nabam  (Dithane  D-14), 
and  Terraclor  (PCNB). 

Terraclor  (PCNB)  may  be  used  as  a 
soil  drench  for  Rhizoctonia  control  on 
living  carnation  plants  at  the  rate  of  1 
to  1%  pounds  of  the  75  per  cent  for- 
mulation per  1,000  square  feet.  When 
used  in  this  way  the  obviously  infected 
plants  should  be  removed  and  the  chemi- 
cal applied  to  an  area  extending  1  to  2 
feet  past  the  infested  area. 

Ferbam,  thiram,  and  captan  are  usu- 
ally applied  at  1  tablespoon  per  gallon 
of  water  at  the  rate  of  V2  to  1  pint  per 
square  foot.  Nabam  may  be  diluted  to 


concentrations  of  1:500  (V2  tablespoon 
per  gallon)  and  applied  at  the  rate  of 
y<2.  to  1  Pmt  Per  square  foot. 

Semesan  at  1  tablespoon  per  gallon 
of  water  is  used  at  the  rate  of  %  Pmt  per 
square  foot.  Do  not  use  Semesan  on 
roses  in  enclosed  or  poorly  ventilated 
areas  since  the  mercury  may  volatilize 
and  cause  severe  injury.  Semesan  may 
be  effectively  used  against  Rhizoctonia 
damping-off  on  such  plants  as  stocks, 
but  plants  such  as  pansy,  petunia,  and 
snapdragon  may  be  stunted  by  its  use. 
Consequently,  be  careful  in  using  Seme- 
san if  the  tolerance  of  the  crop  to  the 
chemical  is  unknown. 

In  using  drenches,  remove  all  obvi- 
ously diseased  plants,  drench  the  in- 
fested area  for  1  to  2  feet  past  the  edges 
of  the  infestation  and  repeat  7  to  10  days 
later. 

Since  nabam  penetrates  deeper  than 
the  solid  materials,  but  has  less  residual 
effect,  a  combination  of  it  with  one  of 
the  other  materials  (for  example,  nabam, 
y±  tablespoon,  plus  captan,  1  tablespoon 
per  gallon  of  water)  may  be  successful. 

It  must  be  remembered  that  these  spot 
treatments  are  not  eradicative  but  tempo- 
rary inhibitory  treatments  at  best.li  soil- 
disease  problems  persist,  they  indicate 
that  some  major  contamination  exists  in 
the  operation  of  the  nursery  and  efforts 
should  be  made  to  see  that  soil,  plants 
and  plant  parts,  and  greenhouse  are 
made  free  of  contaminating  organisms 
(sees.  8  through  13). 


[207] 


Table  17.  Summary  of  Chemicals,  Dosages,  and  Treatments  Used  to 
Control  Diseases  and  Nematodes  in  Nursery  Soils 


Chemical 


Recommendations 


SOIL  FUMIGANTS— FUNGUS  CONTROL 


Chloropicrin 


Formaldehyde 


Methyl  bromide 
(gaseous) 


Terraclor  (PCNB) 


Vapam 


DOSAGE:  Field:  3  cc  per  hole  on  12-in.  centers,  6  in.  deep.  Bulk 
soil :  3-5  cc  per  cu.  ft. 

TREATMENT  PERIOD:  1-3  days,  confine  with  cover,  wet  news- 
papers, or  water  seal. 

AERATION:  7-10days  or  until  all  odor  of  gas  is  gone. Recommended 
for  carnation,  chrysanthemum,  field  soil  applications. 

DOSAGE:  As  drench:  1  pint  in  634  gal-  water,  applied  at  the  rate 
of  Yz  gal.  per  sq.  ft. 

TREATMENT  PERIOD:  24  hr. 

AERATION:  10-14  days.  Stir  thoroughly  and  do  not  plant  until  all 
odor  of  chemical  is  gone. 

DILUTE  METHOD :  2  tbs.  per  %  cup  water.  Sprinkle  this  amount 
over  each  cu.  ft.  of  moist  soil  spread  in  thin  layer,  stacked  and 
covered  for  24  hr.  Sow  seeds  after  24  hr.  and  water  thoroughly. 
Formaldehyde  may  be  used  as  a  drench  for  rooting  beds  and  for 
cleaning  up  greenhouse  areas  when  plants  are  removed.  Chloro- 
picrin or  methyl  bromide  is  better  suited  for  most  nursery  needs. 

DOSAGE :41b. per  100  cu.ft.of  bulk  soil  or  100  sq.ft. of  soil  surface. 

TREATMENT  PERIOD :  24-48  hr.  Must  be  confined  beneath  gas- 
proof cover. 

AERATION :  24-48  hr.  Very  effective  and  especially  suited  for  flat 
and  bulk  soil  fumigation.  Do  not  use  for  carnations  or  chrysan- 
themums. 

DOSAGE:  (for  Rhizoctonia  control)  1-1  y2  lb.  (75%  wettable pow- 
der) per  1,000  sq.  ft. 

TREATMENT  AND  AERATION  PERIOD :  May  plant  immediately 
after  application. 

DOSAGE:  1-2  qt.  per  100  sq.  ft. 

TREATMENT  AND  AERATION  PERIOD:  7-14  days.  Longer 
period  required  if  soil  is  cold  and  wet  after  application. 


GENERAL  RECOMMENDATIONS :  Use  methyl  bromide  for  soil  in  flats  and  containers, 
chloropicrin  or  methyl  bromide  for  bulk  soils,  and  chloropicrin,  methyl  bromide,  or 
Vapam  for  other  soils,  whichever  is  most  suited  to  your  needs. 


208  ) 


Table  17.  (Concluded) 


Chemical 


Recommendations 


SOIL  FUMIGANTS— NEMATODE  CONTROL 


D-D  mixture 


Ethylene  dibromide 


Chloropicrin 


Methyl  bromide 
(gaseous) 


DOSAGE:  200-400  lb.  per  acre. 
TREATMENT  AND  AERATION:  1-2  weeks. 

DOSAGE:  3-6  gal.  per  acre  for  the  85%  EDB.  Dosage  must  be  al- 
tered according  to  the  actual  concentration  of  EDB  in  the  material 
used,  since  it  is  marketed  in  several  forms. 

TREATMENT  AND  AERATION:  2-3  weeks.  Do  not  use  for  car- 
nations. 

This  compound,  when  applied  at  dosages  sufficient  for  fungus 
disease  control,  will  also  kill  nematodes.  Other  nematocides  are 
cheaper,  however. 

When  used  for  fungus  disease  control,  nematodes  are  also  con- 
trolled. It  may  be  used  at  lower  dosage  for  weed  and  nematode 
control.  More  expensive  than  standard  nematocides. 

DOSAGE:  1^-2  lb.  per  100  cu.  ft.  or  sq.  ft. 

TREATMENT :  24-48  hr. 

AERATION :  24-48  hr. 


GENERAL  RECOMMENDATIONS :  If  only  nematode  control  is  desired  use  D-D  mix- 
ture or  ethylene  dibromide,  since  they  are  cheaper  and  very  effective.  Do  not  use  for 
fungus  control. 

SOIL  DRENCHES— SPOT  TREATMENT  FOR  DAMPING-OFF  CONTROL 


Captan,  ferbam, 
thiram 

Nabam 

Semesan 


Terraclor  (PCNB) 


Remove  all  diseased  plants,  drench  area  1-2  ft.  past  the  edges  of 

infestation  and  repeat  7-10  days  later. 
DOSAGE:    1  tb.  per  gal.  water  applied  at  the  rate  of  M>-1  pint  per 

sq.  ft. 

Same  as  above  except  that  dosage  is  M~3^  tb.  per  gal.  Pene- 
trates further  than  solid  suspended  materials  in  soil,  but  has  less 
residual  effect. 

Same  as  above  except  that  dosage  is  1  tb.  per  gal.  water  applied 
at  the  rate  of  ^  pint  per  sq.  ft.  Since  it  contains  mercury,  do  not 
use  near  roses. 

For  Rhizoctonia  control  on  carnation,  remove  all  obviously  infec- 
ted plants,  apply  1-13^2  lb.  (75%  wettable  powder)  per  1,000  sq. 
ft.  to  an  area  extending  1-2  ft.  beyond  infected  plants. 


GENERAL  RECOMMENDATIONS :  All  have  been  used  with  varying  degrees  of  success. 
These  will  not  eradicate  pathogens,  but  they  may  serve  to  check  their  advance. 


[209] 


SECTION 


Treatment  of 
Nursery  Containers 


Kenneth  F.  Baker 


Chester  N.  Roistacher 


Philip  A.  Chandler 


Heat  treatment  of  containers 


Chemical  treatment  of  containers 


.REATMENT   OF   NURSERY   Soil  to   free  it 

of  pathogenic  organisms  is  rapidly  be- 
coming an  accepted  procedure  in  Cali- 
fornia. Such  disinfestation,  either  by 
steam  or  chemicals,  is  best  performed  in 
the  containers  (for  example,  flats,  pots, 
cans,  or  benches)  in  which  the  soil  is  to 
be  used.  There  are  some  situations  in 
which  this  is  impracticable,  however,  and 
soil  is  treated  in  bulk  (sees.  8  through 
11) .  There  must  then  be  some  additional 
means  of  treating  the  containers  if  dis- 


ease control  is  to  be  achieved.  Placing 
treated  soil  in  infested  containers  per- 
mits rapid  extension  of  pathogens  and 
usually  leads  to  severe  disease  losses 
(sees.  3  and  14).  On  the  other  hand,  if 
containers  are  treated,  bulk  soil  may  be 
steamed  and  placed  in  them  with  no 
greater  disease  hazard  than  the  possi- 
bility of  contamination  from  handling. 
Since  such  an  operation  is  commercially 
feasible,  methods  for  container  treat- 
ment are  presented  in  this  section. 


HEAT  TREATMENT  OF  CONTAINERS 


Heat  treatment  of  containers  is  not  ap- 
preciably different  from  the  steaming  of 
soil  discussed  in  Section  8,  either  in 
methods  or  temperatures  required.  After 


CAUTION: 

Many 

of 

the  chemicals 

mentioned 

in  this 

manual   are 

poi- 

sonous   anc 

may 

be 

harmful. 

The 

user  should 

carefu 

lly 

Follow  the 

pre- 

cautions  on 

the   1 

□  be 

Is  of  the 

con- 

tainers. 

steaming,  flats  or  pots  may  be  filled  with 
treated  soil  and  planted  immediately, 
whereas  those  chemically  treated  require 
a  period  of  aeration  before  use.  Steam 
may  also  be  used  without  injury  to  near- 
by living  plants  or  irritation  to  workers, 
in  contrast  to  most  chemicals,  which 
cannot  safely  be  used  in  confined  areas. 
Containers  may  be  placed  in  a  bench  and 
covered  with  a  tarpaulin  beneath  which 
steam  is  released.  Equipment  used  for 
treating  soil  in  containers  (Sec.  10)  may 
also  be  used  for  this  purpose. 


[210] 


Flats 

The  minimum  treatment  should  be  at 
least  180°  F  for  30  minutes,  and  there 
is  no  harm  from  higher  steam  tempera- 
tures. Flats  may  be  steamed  in  stacks 
either  covered  with  heavy  tarpaulins,  or 
placed  in  a  chamber  with  flowing  steam 
or  in  a  pressure  container  (Sec.  10). 
Flats  should  be  separated  from  each 
other  horizontally  by  %-inch  strips,  or 
stacked  in  a  staggered  manner  with  ver- 
tical spaces  of  about  1  inch  (fig.  104). 
This  will  permit  the  free  flow  of  steam, 
and  make  for  faster,  more  economical 
heating.  This  process  can  be  mechanized 
by  stacking  flats  on  pallets  for  handling 
by  fork-lift  tractors  (Sec.  17)  ;  such  a 
practice  would  decrease  the  chance  of 
recontamination  by  keeping  treated  flats 
off  the  ground  and  by  reducing  han- 
dling. In  any  case,  the  pallets  should  be 
treated  before  re-use. 

Continuous  steam  tunnels  have  been 
used  in  canneries  and  fruit  packing- 
houses for  sterilizing  of  lug  boxes  be- 
fore re-use.  In  general,  it  has  been  found 
that  exposure  of  2  minutes  to  flowing 
steam  of  212°  F  is  necessary  to  destroy 
molds  in  corners  of  the  boxes.  Results 
with  nursery  flats  should  be  comparable. 
The  time  might  be  shortened  somewhat 
by  using  superheated  steam.  From  the 
standpoints  of  efficient  use  of  steam  and 
avoiding  moving  parts,  the  use  of  space 
steaming  in  piles  described  above  is  to 
be  preferred. 

Benches 

Benches  are  satisfactorily  treated  by 
steaming  them  under  a  tarpaulin.  As 
with  flats,  the  minimum  treatment  should 
be  180°  F  for  30  minutes. 

It  should  be  pointed  out  that  the  use 
of  intense  localized  heat  on  benches, 
flats,  and  so  on,  may  not  be  satisfactory. 
Sometimes  nurserymen  rapidly  go  over 
their  benches  with  a  blowtorch.  This 
practice  is  of  little  use  because  it  would 
be  necessary  to  char  the  surface  in  order 
to  heat  in  the  cracks. 


Somewhat  better  is  the  use  of  a  jet  of 
flowing  steam  directed  on  the  surface;  if 
continued  long  enough  to  heat  the  ma- 
terial it  is  satisfactory.  So  much  steam 
is  lost  in  this  method,  however,  that  it  is 
prohibitively  expensive.  By  confining 
the  steam,  one  obtains  the  efficient  space 
treatment  described  above. 

Clay  pots 

Empty  clay  pots  present  a  special 
problem  in  California  nurseries.  Water 
is  continually  evaporating  from  the 
porous  surface  and  leaving  a  deposit  of 
soluble  salts  that  in  time  becomes  clearly 
visible  (Sec.  4).  In  most  nursery  soils 
the  roots  tend  to  grow  out  to  the  pot  and 
then  to  spiral,  forming  a  more  or  less 
hollow  cone.  Most  of  the  roots  are  then 
located  in  the  highly  saline  zone,  with 
resultant  stunting  of  plant  growth. 

There  are  three  ways  of  reducing  this 
hazard: 

1.  Using  a  nonporous  container  (for 
example,  cans,  plastic  pots,  or  painting 
the  inside  of  clay  pots)  as  discussed  in 
Section  4.  The  success  of  this  method  is 
shown  by  the  common  use  of  cans  in 
California  nurseries. 

2.  Using  a  light  porous  soil  mix  of  the 
U.  C.  type.  This  provides  favorable  aera- 
tion conditions  for  roots  throughout  the 
soil  mass  instead  of  just  at  the  sides,  as 
occurs  with  heavy  soils  (fig.  61).  Such 
a  low-salinity,  well-aerated  soil  there- 
fore reduces  the  seriousness  of  salt  ac- 
cumulation in  clay  pots,  but  cannot 
eliminate  it. 

3.  Soaking  of  clay  pots  for  24  hours 
or  more  in  water,  then  washing  them  in 
the  usual  way.  We  have  found  that  this 
practice  has  largely  solved  the  salt  prob- 
lem. It  is  possible,  therefore,  to  achieve 
a  reduction  of  salinity  and  to  disinfest 
the  pots  in  one  operation  by  heating  the 
water  in  which  they  are  soaked  to  140° 
to  180°  F.  This  will  destroy  the  algae 
and  mosses,  as  well  as  disease  organisms 
that  commonly  persist  from  one  plant- 


[211] 


ing  to  the  next.  The  pots  may  then  be 
washed  if  desired. 

It  is  considered  that  hot-water  treat- 
ment of  clay  pots  is  preferable  to  steam- 
ing in  California,  because  of  the  salinity 
problem.  If,  however,  the  empty  pots  are 
not  soaked,  they  definitely  should  be 
steamed  before  re-use,  heating  to  180°  F 
for  30  minutes.  They  can  be  efficiently 


handled  when  nested  in  horizontal  rows, 
and  may  be  handled  on  a  pallet  equipped 
with  sideposts  (fig.  134). 

Metal  or  plastic  containers 

Metal  or  plastic  containers  that  are  so 
tapered  that  they  nest  may  be  steamed 
but  must  be  stacked  vertically;  it  is  un- 
necessary to  soak  them,  however. 


CHEMICAL  TREATMENTS  OF  CONTAINERS 


Containers  disinfested  with  some 
chemicals  must  be  aerated  to  dispel  the 
materials  before  use.  With  some  ma- 
terials (for  example,  methyl  bromide 
and  copper  naphthenate)  the  delay  is 
short  and  reasonably  convenient,  but 
with  others  (for  example,  formaldehyde) 
it  may  last  for  several  days.  Some  of 
these  materials  are  harmful  to  plants  and 
annoying  to  workmen,  and  must  be  han- 
dled accordingly. 

Copper  naphthenate  is  retained  by 
wood  for  a  year  or  more  and  thus  pro- 
vides a  self-disinfesting  surface. 

Methyl  Bromide 

Methyl  bromide  is  used  at  the  same 
dosage,  4  pounds  per  100  cubic  feet  of 
space,  and  in  the  same  way  as  for  soil 
(Sec.  11).  Containers  should  be  aerated 
for  1  day  before  use.  This  is  a  very  satis- 
factory method  for  the  nursery  with 
types  of  steaming  equipment  that  make 
no  provision  for  container  treatment.  It 
is  of  limited  value  to  a  nursery  that  uses 
chemicals  for  soil  treatment,  since  the 
soil  should  in  such  cases  be  treated  after 
being  placed  in  the  container.  Methyl 
bromide  is  expensive  to  use  for  empty 
flats  or  pots  because  the  dosage  is  the 
same  as  if  they  were  filled  with  soil;  see 
Section  1 1  for  approximate  costs.  This 
material  effectively  disinfests  the  flats  or 
pots,  but,  unlike  copper  naphthenate, 
leaves  no  residue  that  will  reduce  recon- 
tamination. 


In  New  York  it  has  been  found  (Lear 
and  Mai,  1952)  that  methyl  bromide  at 
4.6  pounds  per  100  cubic  feet  of  space 
may  be  used  to  treat  bales  of  burlap 
bags,  farm  machinery,  and  other  equip- 
ment to  free  them  of  nematodes.  Trucks 
were  covered  with  polyethylene  tarps 
and  treated  without  sustaining  damage. 
It  is  desirable  first  to  wash  dried  mud 
from  under  fenders  and  wherever  it  has 
accumulated. 

Formaldehyde 

Formaldehyde  is  an  excellent  fungi- 
cide, although  an  obnoxious  one  and 
therefore  not  frequently  used  for  con- 
tainer treatment.  It  is  used  at  a  dilution 
of  1  gallon  of  commercial  formaldehyde 
(37  per  cent  concentration)  to  18  gal- 
lons of  water.  Application  is  made  by 
dipping  or  spraying  flats,  pots,  or  other 
containers.  They  may  be  immersed  for  a 
few  minutes  in  the  material,  permitted 
to  stand  until  the  excess  has  drained 
back  into  the  tank,  then  stacked.  For- 
maldehyde in  the  above  concentration 
may  be  sprayed  on  containers  until  they 
are  thoroughly  wetted,  using  a  very 
coarse  nozzle  to  minimize  volatilization. 
In  either  case  the  containers  should  be 
stacked  while  still  wet,  and  preferably 
covered  with  a  tarp  for  24  hours.  When 
they  are  uncovered  they  should  not  be 
permitted  to  become  dry  at  any  time 
until  the  odor  of  formaldehyde  is  gone 
(usually  less  than  5  days).  The  water  in 


[212] 


a  formaldehyde  solution  will  evaporate 
before  the  formaldehyde  volatilizes,  and 
the  chemical  then  passes  over  to  a  white 
powder,  paraformaldehyde.  From  this 
state  it  volatilizes  at  a  much  slower  rate 
than  from  a  water  solution.  For  example, 
flats  kept  wet  were  free  of  formaldehyde 
in  4  to  5  days,  whereas  those  permitted 
to  dry  were  found  to  require  10  days  to 
reach  comparable  levels.  It  should  be 
noted  that  the  rate  of  volatilization  from 
paraformaldehyde  may  be  so  low  as  not 
to  be  detected,  but  that  if  the  flats  are 
again  wetted  it  may  appear  in  amounts 
toxic  to  plants. 

It  is  a  wise  precaution  to  check  care- 
fully for  the  presence  of  formaldehyde 
before  planting  flats.  To  do  this,  wrap 
moist  sample  flats  in  a  piece  of  poly- 
ethylene plastic  for  24  hours,  then  open 
one  side  to  see  whether  the  odor  of  the 
fumes  can  be  detected.  An  even  safer 
method  is  to  fill  several  flats  with  soil, 
plant  sensitive  seedlings  (for  example, 
petunia)  in  them,  then  cover  with  a  poly- 
ethylene tarp  for  24  hours.  Injury  will 
be  evident  on  seedlings  near  the  sides 
of  the  flat  within  2  days  as  white  dead 
areas  in  the  leaves. 

The  highly  irritating  nature  of  for- 
maldehyde fumes  makes  indoor  use  ob- 
jectionable. It  should  never  be  used  in 
the  same  room  with  living  plants.  Con- 
tainers are  best  treated  and  aired  out- 
doors, down-wind  from  the  growing 
areas.  Workers  should  remain  up-wind 
from  the  material,  and  in  still  weather  a 
fan  might  be  used.  They  should  wear 
rubber  gloves  when  handling  the  treated 
flats,  and  tight  goggles  when  spraying 
the  chemical  on  flats.  Use  of  a  full-face 
gas  mask  may  be  desirable. 

Formaldehyde  is  extremely  useful  as 
a  rapid  disinfestant  for  tools  (Sec.  3). 
If  it  is  used  on  benches,  lath  shade- 
frames,  or  glasshouses,  the  same  pre- 
cautions regarding  application,  keeping 
wet,  and  aeration  should  be  observed  to 
prevent  injury  to  plants  or  workers. 


It  should  be  noted  that  formaldehyde, 
like  methyl  bromide,  leaves  no  perma- 
nent residue  that  will  reduce  recontami- 
nation. 

Formaldehyde-steam 

A  southern  California  mushroom 
grower  has  successfully  used  formalde- 
hyde-steam mixtures  (Sec.  10,  type  26) 
for  disinfesting  his  houses.  This  tech- 
nique is  worthy  of  trial  for  treating 
glasshouses  between  crops. 

Copper  Naphthenate 

Methods  of  application 

Copper  naphthenate  is  an  excellent 
material  for  treatment  of  wooden  con- 
tainers. It  prevents  fungi  from  growing 
on  or  into  wood  which  it  thus  protects 
from  decay,  as  well  as  rendering  the  con- 
tainers self-disinfesting  for  a  time.  It  is 
available  in  bulk  as  a  concentrate  having 
8  per  cent  copper  (see  Appendix)  that 
may  be  diluted  with  Stoddard  solvent  (1 
gal.  of  the  napthenate  to  3  gal.  of  sol- 
vent) to  a  concentration  of  2  per  cent 
copper.  This  is  applied  by  dipping  for 
5  to  30  minutes  either  the  finished  flats 
or  the  shook  for  making  them,  in  a  tank 
of  the  chemical.  The  excess  is  drained 
back  into  the  tank,  and  the  containers 
then  aerated  for  a  day.  The  material  may 
also  be  brushed  on.  A  gallon  of  material 
covers  from  200  to  400  square  feet  of 
surface  by  either  method  of  application, 
according  to  the  smoothness  of  the  wood 
treated.  Diluted  copper  naphthenate  sold 
under  several  trade  names  may  be  much 
more  expensive  than  the  above  but 
equally  satisfactory. 

The  corrosive  effect  of  treated  wood 
on  iron  or  galvanized  nails  has  been 
eliminated  by  using  aluminum  nails  in 
constructing  flats,  benches,  and  so  on. 

Effectiveness 

Of  the  materials  presently  available, 
copper   napthenate   most   nearly   fulfills 


[213] 


B 


\v 


Mi  Mat  •*  ;• 


[214] 


the  requirements  of  a  residual  flat  disin- 
festant.  It  kills  pathogens  near  or  in 
contact  with  the  wood  by  slowly  leaching 
into  the  soil  for  a  distance  of  1  to  2 
inches  from  the  wood.  Although  it  is 
injurious  to  roots  that  enter  this  zone, 
for  many  nursery  purposes  it  is  reason- 
ably satisfactory.  It  is  still  effective  after 
a  year's  contact  with  moist  soil.  That  it 
is  relatively  inexpensive  is  shown  by  the 
fact  that  nurserymen  have  found  it 
economically  justified  even  when  used 
solely  as  a  wood  protective. 

In  repeated  tests,  treated  flats  have  Dot 
carried  damping-off  fungi  from  a  plant- 
ing of  infested  soil  to  a  subsequent  one 
of  steamed  soil  'fig.  108,  A  I .  even 
though  emptied  and  refilled  six  times. 
The  flats  should  be  rinsed  lightly  with 
water  before  re-using,  in  order  to  re- 
move the  attached  soil. 

Injury  to  seedlings 

Injury  to  roots  that  enter  the  zone 
usually  is  seen  as  darkening,  or  even 
killing  of  the  tips  i  fig.  108.  B  i .  Such  in- 
jury occurred  on  seedling  roots  of  all 
plants  tested,  but  some  of  them  I  pepper. 
pea.  nasturtium,  calendula '  show  no 
corresponding  injury  to  top  growth. 
Others  |  tobacco,  larkspur.  Iceland 
poppy,  snapdragon,  petunia,  pansy.  Lo- 
belia. Coreopsis.  }<emesia.  China  aster, 
and  stock  >  show  varying  degrees  of  in- 
jury around  the  edges  of  the  flats  i  fig. 
108.  C). 

It  is  apparent  that  copper  napthenate 
is  somewhat  more  soluble  than  necessary 
to  achieve  the  self-disinfesting  properly; 
on  the  other  hand.  Wolman  salts  and 
Erdalith  •  see  below  i  are  too  insoluble. 
Perhaps  an  intermediate  level  of  solubil- 


ity mav  later  be  achieved  that  will  be 
self-disinfesting  but  less  injurious  to 
roots.  Tests  have  shown  that  the  Stod- 
dard solvent  is  not  the  source  of  residual 

toxicity,  for  it  volatilizes  rapidly. 

Uses 

It  should  be  emphasized  that  many 
growers  are  unaware  of  the  injurious 
effect  of  this  material,  and  that  it  should 
be  decided  in  each  instance  whether  its 
usefulness  exceeds  its  injuriousness.  As 
a  suggestion,  the  following  uses  may  be 
cited. 

Copper  naphthenate  may  safely  be 
used  on  benches  and  shelves  on  which 
flats  or  other  containers  are  to  be  placed. 

It  is  excellent  for  treating  timbers  to 
be  placed  on  the  ground  for  supporting 
flats  I  Sec.  3  I . 

It  is  verv  useful  for  disinfecting  a 
glasshouse  bench  before  filling  it  with 
soil,  under  conditions  where  living 
plants  must  remain  in  the  house.  If  the 
ventilators  are  kept  open  until  the  Stod- 
dard solvent  has  evaporated  1 12  to  24 
hours  |  there  is  little  danger  to  surround- 
ing plants.  It  should  be  noted  that  some 
sensitive  plants,  such  as  maidenhair 
fern,  may  be  injured  by  the  volatile  sol- 
vent under  conditions  of  inadequate 
ventilation.  The  copper  naphthenate  resi- 
due is  not  volatile  and  does  not  injure 
adjacent  plants. 

The  chemical  mav  safely  be  used  on 
flats  or  benches  which  are  to  be  filled 
with  soil  and  used  for  large  plants,  or  in 
which  seedlings  are  planted  2  to  3  inches 
from  the  sides. 

It  should  not  be  used  on  seed  flats  or 
on  containers  in  experimental  work. 
Apparently  it  is  toxic  to  large  geraniums 


Fig.  108.  Effect  of  copper-naphthenate  treatment  of  flats  on  carryover  of  damping-off  fungi, 
and  on  seedlings  grown  in  them.  Untreated  flats  at  left,  treated  at  right  in  each  case.  Steamed 
soil  used  in  all  cases.  A,  Effect  on  carryover  of  R"  zcc^cr'a  on  a  flat  from  a  previous  diseased 
planting,  as  shown  by  damping-off  of  pepper  seedlings.  Note  elimination  of  marginal  damping- 
off  by  t-eatment.  B,  injury  to  pepper  root  tips  from  a  flat  planted  with  four  changes  of  soil  in 
4  months  since  treatment.  C,  Toxicity  to  tobacco  seedlings  from  a  flat  treated  and  aerated  for 
10  days  before  seeding;  half  flats  are  shown. 


[  215  ] 


grown  in  clay  pots  treated  with  it,  and 
its  use  on  such  containers  is  inadvisable. 

Other  Materials 

Several  chemicals  have  been  used  at 
one  time  or  another  for  container  treat- 
ment, without  general  adoption.  Among 
these  are  the  following. 

Ethylene  oxide  gas  (Carboxide: 
contains  10  per  cent  ethylene  oxide  in 
90  per  cent  carbon  dioxide)  used  at  0.5 
pound  of  active  ingredient  per  100 
cubic  feet  has  been  effective  in  destroy- 
ing the  bacteria  that  cause  potato  ring 
rot  in  16  hours  in  fumigation  chambers. 
It  has  also  been  used  for  soil  at  some- 
what higher  dosages. 

Sodium     hypochlorite     solutions 


have  been  successfully  used  for  disin- 
festation  of  bulb  trays,  fruit  boxes,  and 
similar  equipment.  Commercial  solu- 
tions (for  example,  Clorox,  Purex) 
usually  have  about  5  per  cent  available 
chlorine,  and  are  diluted  to  0.4  per  cent 
(1  quart  to  3%  gallons)  or  0.2  per  cent 
(1  quart  to  61/4  gallons) .  They  are  either 
used  as  dips  or  sprayed  on  the  containers, 
and  may  be  covered  with  a  polyethylene 
tarpaulin  for  24  hours. 

Fixed  wood  preservatives  such  as 
Wolman  salts,  Erdalith,  and  Celcure  are 
so  firmly  stabilized  in  the  wood  as  to  be 
valueless  in  producing  self-disinfesting 
flats.  They  are,  however,  excellent  wood 
preservatives  and  are  not  generally 
harmful  to  plant  roots. 


[216] 


SECTION 


Development  and 
Maintenance  of  Healthy 
Planting  Stock 


Kenneth  F.  Baker 


Philip  A.  Chandler 


Importance  of  clean  propagating  material 


How  to  obtain  clean  seed  or  stock 


Maintaining  clean  stock 


I 


T  has  already  been  pointed  out  I  See. 
3  i  that  plant-disease  organisms  may  get 
into  a  planting  from  I  1  |  the  soil.  I  2  I 
the  seeds,  cuttings,  bulbs,  or  other 
propagative  stock,  or  1 3 1  from  con- 
taminated containers  or  tools,  or  from 
infected  material  splashed,  blown,  or 
otherwise  dispersed.  Soil  infestation  is 
discussed  in  sections  3.  14.  and  15,  and 
methods  for  controlling  it  in  sections  3 
and  8  through  11.  The  recontamination 
problem  and  its  control  are  considered 
in  sections  1,  3.  12.  and  14.  There  re- 
mains the  propagative  material  to  be 
discussed  in  this  section,  explaining  how 
such  stock  may  be  produced  and  used, 
and  the  benefits  to  be  derived  from  us- 
ing it. 

Almost  anyone  associated  with  the 
nursery  business  will  agree  that  patho- 
gen-free planting  stock  is  desirable. 
There  may  be  differences  of  opinion  on 
methods  for  producing  such  material, 
the  price  one  can  justifiably  pav  for  it. 


and  the  best  ways  to  use  it  in  commercial 
growing.  The  proper  use  of  clean  seed 
and  stock  probably  will  be  profitable 
anytime,  and  might  prevent  bankruptcy. 
That  we  fall  far  short  of  the  ideal  in 
production  and  utilization  of  such  stock 
is  shown  bv  the  rapid  increase  in  fungi- 
cide sales  to  nurseries  in  recent  years. 

Two  types  of  nurservmen  are  inter- 
ested in  pathogen-free  planting  stock: 
(1)  one  who  produces  plants  or  flowers 
for  market.  He  wishes  clean  stock  as  a 
means  toward  an  end,  the  cheapest,  most 
dependable  possible  production  of 
plants,  \\hen  possible,  clean  stock  should 
be  obtained  by  such  a  nurseryman  from 
a  specialist  propagator.  L  nf  ortunatelv. 
pathogen-free  stock  of  onlv  a  few  crops 
is  presently  available,  and  the  grower 
must  usually  develop  his  own  supply 
along  the  lines  discussed  here.  This  sec- 
tion is  presented  primarily  to  assist  such 
a  grower.  As  he  begins  to  provide  clean 
stock  to   others  he   often   gradually  be- 


[217] 


comes  a    (2)    specialist   propagator,   to  cultural  operation,  most  of  which  must 

whom  the  production   of  pathogen-free  be  locally  evolved  as  needed.  Therefore, 

stock  is  the  end  desired.  This  business  it  is  expected  that  this  section  will  be 

involves  much  highly  specific  informa-  less  useful  at  the  specialist  level  than  for 

tion    for   the    given   crop,    disease,    and  the  general  grower. 


IMPORTANCE  OF  CLEAN  PROPAGATING  MATERIAL 


Only  a  small  percentage  of  seed  or 
propagative  stock  ordinarily  is  infested, 
but  this  is  of  the  greatest  importance 
because  it  serves  as  a  center  of  infection. 
Whether  the  pathogen  spreads  with  soil, 
as  in  the  case  of  most  of  the  fungi  con- 
sidered in  this  manual,  or  by  spores,  the 
end  result  is  to  start  a  series  of  "spot 


ires. 


Rapid  increase  of  pathogens 

Other  conditions  being  equal,  the 
greater  the  number  of  spores  produced 
by  a  fungus,  or  eggs  by  a  nematode,  the 
more  effective  and  rapid  is  its  spread. 
Although  the  dissemination  process  is 
prodigiously  wasteful,  its  effectiveness 
cannot  be  doubted.  From  a  single  sclero- 
tium  of  the  cottony-rot  fungus  (Sclero- 
tinia  sclerotiorum)  are  produced  several 
tiny,  cup-shaped  structures  said  to  form 
as  many  as  310,000,000  spores.  A  single 
leaf  spot  of  celery  Septoria  late  blight 
may  produce  more  than  200,000  spores. 
A  single  female  root-knot  nematode  com- 
monly lays  500  to  1,000  eggs  and  may 
even  reach  2,800;  a  single  gall  may  con- 
tain many  such  adults.  Snapdragon  rust 
has  been  conservatively  estimated  to  be 
able  to  increase  from  a  single  rust  pus- 
tule in  a  shipment  of  10,000  plants  to 
an  average  of  4,600  lesions  on  each  plant 
90  days  later.  The  grower  who  believes 
that  diseases  "suddenly  appear"  is  ob- 
serving only  the  final  stages  in  their 
build-up.  Because  of  such  rapid  increase 
it  is  particularly  important  that  initial 
infections  be  prevented.  Epidemics,  like 
fires,  are  best  stopped  while  they  are 
small. 


Importance  of 
initial  infections 

The  relative  importance  of  each  infec- 
tion is  determined  by  its  newness  in  that 
location,  and  by  its  permanence.  The 
ascending  progression  of  danger  may  be 
given  as  follows: 

1.  Least  important  are  those  organ- 
isms that  are  already  present  in  the  area 
and  do  not  infest  soil  (for  example, 
snapdragon  rust  in  California).  The 
centers  of  disease  serve  as  temporary 
foci  of  infection  and  may  be  controlled 
or  reduced  in  severity  by  fungicidal 
treatment. 

2.  Somewhat  more  serious  are  organ- 
isms already  present  in  an  area  and  able 
to  infest  soil.  If  the  organism  infests  the 
land  for  only  a  few  years,  as  with  Alter - 
naria  disease  of  zinnia,  bacterial  blight 
of  stock,  and  bacterial  stem  rot  of  del- 
phinium, it  is  bad  enough. 

3.  If,  however,  the  pathogen  is  present 
and  remains  permanently  in  the  soil,  as 
with  Fusarium  wilt  of  aster,  Phytoph- 
thora  root  rot  of  heather,  and  Rhizoc- 
tonia  diseases,  it  is  a  serious  matter. 

4.  Those  organisms  introduced  to  new 
areas  but  not  infesting  soil  (for  example, 
the  appearance  in  1952  of  snapdragon 
rust  in  Australia)  may  create  much  ex- 
citement and  become  very  important. 

5.  A  still  worse  situation  is  the  intro- 
duction to  a  new  area  of  an  organism 
that  will  persist  in  the  soil.  As  in  2  above, 
this  is  serious  if  the  organism  infests 
soil  only  temporarily  (for  example,  mov- 
ing crown-gall  infected  cuttings  into 
virgin  land) . 


[218] 


6.  When  the  new  pathogen  perma- 
nently infests  the  land,  as  with  the  intro- 
duction of  Phytophthora  cinnamomi  on 
young  heather  plants  from  southern  to 
central  California,  the  worst  situation  of 
all  is  reached. 

The  organisms  discussed  in  this 
manual  fall  into  categories  3  and  6,  de- 
pending on  whether  or  not  they  are  new 
to  an  area.  Since  not  all  types  of  a  given 
fungus  are  identical  in  response  to  either 
environment   or  host  plants    (Sec.   15), 


we   are  here  concerned   almost  entirely 
with  category  6,  the  worst  of  all. 

Benefits  from  use 

The  practical  benefits  from  using 
clean  propagating  stock  are  essentially 
those  outlined  for  the  complete  disease- 
control  program  (sees.  1,  2,  3,  16,  and 
17).  Among  these  benefits  are  reducing 
production  cost  and  possibly  reducing 
competition,  as  well  as  aiding  easier, 
more  certain,  less  expensive  production. 


HOW  TO  OBTAIN  CLEAN  SEED  OR  STOCK 


The  first  step  in  a  program  of  this  sort 
is  to  obtain  the  initial  healthy  seed  or 
stock.  In  some  cases  (for  example,  seed 
of  garden  stock;  chrysanthemum  cut- 
tings) these  can  be  purchased  from  a 
specialist  grower  who  maintains  them. 
In  most  instances,  however,  the  nursery- 
man must  still  develop  his  own  clean 
stock.  The  methods  used  in  either  case 
are  outlined  here,  with  details  of  the 
second  types.  Since  recommendations 
could  not  be  given  for  each  of  the  many 
nursery  crops,  even  if  they  had  b:en 
studied,  the  various  methods  are  pre- 
sented, with  applications  indicated  for 
some  specific  plants. 

Man's  experience  with  his  own  fal- 
libility has  often  caused  him  to  provide 
more  than  a  single  defense  against  ad- 
versity, as  insurance  companies  are 
aware.  This  also  applies  to  disease  con- 
trol. Thus,  a  grower  may  discard  all 
obviously  diseased  stock,  treat  the  re- 
mainder, avoid  overhead  sprinkling  or 
other  cultural  practice  that  favors  the 
pathogen,  remove  any  diseased  plants 
that  appear  in  order  to  reduce  inoculum, 
and  spray  regularly  to  protect  against 
any  spores  that  blow  in.  Rarely  is  a 
single  control  measure  sufficient  to  pre- 
vent a  plant  disease.  This  is  true  of  the 
nursery  diseases  discussed  in  this 
manual,  and  particularly  so  in  the  de- 


velopment and  production  of  pathogen- 
free  stock.  Hence  several  concurrent 
procedures  are  often  suggested  to 
achieve  this  objective. 

If  a  few  healthy  plants  can  be  found, 
the  grower  can  propagate  from  these  by 
observing  reasonable  sanitary  proce- 
dures (Sec.  1).  These  often  cannot  be 
found,  however,  and  the  grower  must 
resort  to  other  more  involved  techniques. 

Plant  Grows  away  from  the 
Pathogen 

It  is  possible  in  some  cases,  by  mani- 
pulation of  the  environment  or  cultural 
practices,  to  obtain  clean  stock  from 
diseased  plants.  The  plant  may  be  grown 
in  such  a  way  that  the  propagative  parts 
remain  uninfected  because  they  develop 
in  a  position  and  under  conditions  un- 
favorable to  the  disease. 

Tip  cuttings 

Thus,  it  is  possible  to  reduce  (but  not 
commercially  to  eliminate)  V erticillium 
wilt  from  chrysanthemum  by  taking  tip 
cuttings  from  rapidly  growing  shoots  at 
least  12  inches  above  the  ground.  If  each 
cutting  is  then  established  in  a  separate 
pot  of  steamed  soil,  grown  to  maturity, 
only  healthy  plants  used  in  propagation, 
and  the  process  repeated,  it  should  be 
possible  to  derive  clean  stock.  It  is  now 


[219] 


done  faster  and  more  reliably  by  "Cul- 
turing  Methods"  (see  below).  With 
fungi  (for  example,  Rhizoctonia  and 
water  molds)  that  do  not  invade  the 
water-conducting  system  (as  do  Verti- 
cillium  and  Fusarium)  this  is  more 
easily  accomplished. 

It  is  possible1  to  get  Choisya  cuttings 
free  from  Phytophthora  by  taking  them 
from  large  shrubs  at  a  point  at  least  4 
feet  from  the  ground,  and  by  rooting  and 
growing  them  in  steamed  media  to  pro- 
duce gallon-can  stock  without  disease 
loss.  This  is  in  striking  contrast  to  the 
50  to  90  per  cent  mortality  in  many  nurs- 
eries. Furthermore,  these  plants  have 
grown  exceptionally  well  when  planted 
in  gardens.  It  is,  of  course,  necessary 
that  such  cuttings  be  taken  in  the  dry 
season,  that  the  shrubs  either  not  be 
splashed  with  mud  or  debris  by  rains, 
down-spouts,  sprinklers,  or  other  ways, 
or  that  the  shrubs  be  so  high  that  such 
spattering  does  not  occur  at  the  levels 
where  cuttings  are  taken.  It  is  further 
necessary  that  cuttings  be  placed 
on  clean  papers  or  in  steamed  baskets, 
never  on  the  ground,  to  avoid  con- 
taminating them.  Cuttings  should  not  be 
taken,  unless  it  is  absolutely  imperative 
to  do  so,  from  known  infected  plants  or 
infested  areas.  These  same  conditions 
apply  equally  to  all  other  types  of  plant- 
ing material  discussed  herein. 

Similarly,  the  taking  of  azalea  and 
camellia  cuttings  from  high  shrubs 
rather  than  low  plants  6  to  12  inches 
from  the  ground,  as  is  so  often  done, 
would  greatly  reduce  carryover  of 
Rhizoctonia  and  water  molds.  A  similar 
method  has  been  used  in  Oregon  to  ob- 
tain strawberry  plants  free  of  the  Phy- 
tophthora that  causes  red  stele. 

The  same  method  would  free  heather 
cuttings  of  Rhizoctonia  and  of  Phytoph- 

1  Demonstrated  in  1954  by  J.  A.  Bcutel  (As- 
Bistanl  Agriculturist,  Farm  Advisor's  Office,  Los 
Angeles  County)  in  tin-  Department  of  Plant 
Pathology,  University  of  California,  Los  An- 
geles. 


thora  cinnamomi,  cause  of  one  of  the 
principal  diseases  of  the  crop. 

Application  of  this  general  system 
could  also  provide  a  source  of  planting 
stocks  free  from  root-knot  nematode  and 
the  oak-root  fungus. 

By  such  careful  manipulations,  and 
strict  avoidance  of  overhead  watering, 
syringing,  or  spraying,  it  should  be  pos- 
sible to  obtain  cuttings  of  marguerite 
daisies,  shrubs,  and  other  plants,  free  of 
crown  gall,  and  oleanders  free  of  stem 
and  leaf  gall. 

Araucaria  tip  cuttings  taken  from 
plants  grown  to  a  height  of  4  to  5  feet 
under  conditions  free  from  splashing, 
would  be  uncontaminated  by  Rhizoc- 
tonia. 

It  is  worth  an  attempt  to  develop  a 
nucleus  stock  of  Esther  Read  daisy  free 
of  bacterial  fasciation  by  (1)  training 
up  the  shoots,  (2)  scrupulously  avoiding 
sprinkling,  syringing,  or  spraying  of  the 
plants,  (3)  taking  tip  cuttings,  and  (4) 
rooting  and  growing  them  in  individual 
pots  free  from  all  water  splashing. 

Plants  on  frames 

A  modification  of  this  is  to  train  cane- 
forming  plants,  such  as  Dieffenbachia 
and  some  Philodendrons,  so  that  the  tips 
are  taken  several  feet  above  the  ground. 
This  will  free  them  of  Rhizoctonia  and 
water  molds,  and  if  overhead  watering 
and  high  humidity  are  avoided,  will 
eliminate  bacterial  soft  rot  as  well. 

Peperomia  may  easily  be  freed  from 
Rhizoctonia  and  water  molds  by  taking 
tip  cuttings  from  high  on  the  plant.  We 
easily  freed  fuchsia  and  coleus  plants 
from  foliar  nematode  by  growing  them 
in  a  glasshouse  without  wetting  the 
foliage,  even  when  the  diseased  leaves 
were  not  removed,  as  they  should  have 
been.  Tip  cuttings  taken  from  these 
plants*  6  to  12  inches  above  infected 
leaves  remained  healthy. 

By  growing  on  wooden  or  wire  up- 
rights or  frames  to  get  the  plants  as  far 
off  the  soil  as  possible,  and  avoiding  all 


[220] 


splashing  water,  it  is  easy  to  free  such 
trailing  plants  as  Pellionia,  Fittonia, 
Nephthytis,  Philodendron  cordatum, 
and  ivy  from  Rhizoctonia  and  water 
molds.  Similarly,  ivy  may  also  be  freed 
of  bacterial  leaf  spot.  It  would  seem  a 
reasonable  precaution  to  continue  to 
raise  all  mother  stocks  of  these  plants 
in  this  way. 

Aseptic  culturing 
of  growing  point 

A  further  refinement  is  to  make  tiny 
cuttings  of  the  growing  tip,  to  graft  the 
tips  onto  healthy  plants,  or  to  aseptically 
culture  the  tiny  apical  growing  point 
(much  as  plant  breeders  culture  em- 
bryos) in  order  to  free  the  plant  of 
viruses.  Holmes  (1955,  1956a,  19566) 
and  Martin  (1954)  have  thus  freed 
dahlia  of  the  spotted-wilt  virus,  chrysan- 
themum of  the  aspermy  virus,  and  sweet 
potato  of  another  virus.  Norris  (1954) 
also  eliminated  virus  X  from  potato  in 
this  way.  The  technique  might  well  be 
extended  to  other  kinds  of  plants  by  a 
trained  specialist.  It  is  based  on  the 
fact  that  some  viruses,  under  certain  en- 
vironmental conditions,  move  slowly 
through  tissue  and  may  not  reach  the 
growing  point.  Quak  (1957)  freed  car- 
nations of  viruses  by  culturing  apical 
meristems  grown  under  high  tempera- 
tures. 

Environmental  control 

In  these  techniques  it  is  desirable  to 
make  the  environment  as  unfavorable  as 
possible  for  the  pathogen  without  unduly 
checking  the  growth  of  the  host.  Rhizoc- 
tonia and  water  molds,  under  moderately 
dry  conditions,  are  unable  to  spread 
much  above  the  soil  surface.  Tempera- 
ture control  might  increase  this  differen- 
tiation between  pathogen  and  plant. 

Culturing  Methods 

This  method  apparently  was  first  sug- 
gested by  A.  W.  Dimock  of  Cornell  Uni- 
versity in  1943  against  V erticillium  wilt 


of  chrysanthemum,  and  was  soon 
adopted  by  a  commercial  propagator. 
In  1949  nearly  26!/2  million  mum  cut- 
tings were  produced  in  Ohio,  69.5  per 
cent  of  the  national  total,  largely  by  this 
concern.  This  technique  is  best  per- 
formed by  the  specialist,  although  it  is 
now  used  by  a  number  of  growers  in 
various  parts  of  the  country.  It  has  also 
been  adapted  for  use  in  securing  Verti- 
cillium-iree  sticks  of  rose  budwood  by 
Wilhelm  and  Raabe  (1956),  pathogen- 
free  carnation  stock  by  Tammen,  Baker, 
and  Holley  (1956),  and  pathogen-free 
geranium  stock  by  Munnecke  (1956). 
It  will  undoubtedly  be  found  useful  for 
many  other  crops  grown  in  nurseries. 
Although  it  is  best  adapted  to  use 
against  pathogens  of  either  the  systemic 
or  vascular  type,  it  can  be  used  on  a 
wide  variety  of  plants  and  diseases. 

As  is  often  the  case,  the  method  is 
basically  simple,  but  becomes  complex 
from  the  number  of  unusual  circum- 
stances that  may  arise.  Briefly,  the 
method  (fig.  109)  is  as  follows:  Each 
4-inch  cutting  (fig.  109,  A)  and  a  cor- 
responding tube  of  nutrient  agar  are 
assigned  a  number.  A  1-inch  piece  (B) 
is  cut  from  the  base  of  the  cutting  and 
immersed  in  a  Clorox  solution  (1  volume 
to  4  of  water)  for  at  least  1  minute  (C) . 
The  scalpel  is  flamed  between  cuts.  The 
piece  is  placed  on  a  paper  towel  to  drain 
(D)  for  a  few  minutes,  and  the  basal 
!/4  inch  is  removed.  Four  slices  each 
y%2  inch  thick  are  then  cut  off  and  re- 
tained; the  remaining  ends  are  dis- 
carded. The  stem  is  cut  on  the  paper  towel 
with  a  flamed  scalpel. The  four  thin  slices 
are  transferred  to  a  numbered  agar  slant 
(or  tube  of  broth,  if  bacteria  are  in- 
volved) and  held  at  75°  to  80°  F  (E) . 
The  cuttings  are  then  held  in  polyethy- 
lene bags  in  cold  storage,  or  planted  in 
paper  cups  of  moist  sterile  media  (F)  for 
10  days  or  more  until  readings  are  taken 
on  the  tubes.  If  any  of  the  four  pieces  in 
a  tube  show  any  fungus  or  bacterial 
growth,  the  corresponding  cutting  is  de- 


[221] 


^9 


"73 


Fig.  109.  Culturing   method  for  selecting   chrysanthemum  cuttings  free  of  Verticillium.   See   p. 
221  and  222  for  explanation. 


stroyed.  The  remaining  healthy  cuttings  have  infested  the  soil  and  infected  neigh- 

(G),  when  rooted  and  grown  on,  become  boring  plants. 

the  nucleus  block.  The  cuttings  produced  Modifications  of  this  culturing  process 

by  this  block  are  grown  in  an  increase  have  been   introduced.   Plain   agar  plus 


block,  from  which  cuttings  are  marketed 
without  additional  culturing. 

The  principle  is  to  determine  rapidly 
whether  any  fungus  or  bacterium  is 
present  thai  might  not  normally  be  de- 
ter led  until  later,  when  it  would  already 


sterile,  dried,  chopped  plant  tissue  may 
be  used  instead  of  nutrient  agar  when 
bacteria  are  not  involved.  This  is  par- 
ticularly useful  for  detecting  Verticil- 
lium. Petri  dishes,  rather  than  tube 
slants,  are  then  used. 


222  | 


Heat  Treatment  of  Planting  Stock 

This  method,  introduced  more  than 
70  years  ago  in  Denmark,  has,  except 
for  cereal  seeds,  only  recently  come  into 
commercial  application.  Its  effectiveness 
is  based  on  the  principle  that  some  or- 
ganisms are  more  easily  killed  by  heat 
than  is  the  crop  plant.  Heat  has  been  ap- 
plied through  the  agency  of  water,  dry 
air,  moist  air,  and  carbon  tetrachloride. 
At  present,  water  is  the  best  for  rapid 
treatments  since  its  heat  conductivity  is 
more  than  twenty  times  that  of  dry  air. 
The  use  of  carbon  tetrachloride  is  still 
experimental.  For  long  treatments  (for 
example,  with  some  virus  diseases),  ex- 
posure to  moist  or  dry  air  is  most  useful. 
Neither  all  kinds  of  plants  nor  all  their 
parts  (for  example,  seeds  vs.  leaves)  are 
equally  heat-susceptible,  nor  are  all  types 
of  growth  (for  example,  lush  succulent 
tops  vs.  hard  woody  stems).  The  "trick" 
for  successful  treatment  often  lies  in  se- 
lecting the  most  heat-tolerant  host  tissue, 
and  manipulating  culture  procedures  so 
as  to  produce  the  most  resistant  growth. 
Failure  to  consider  these  basic  facts  has 
led  to  many  "successful  operations" 
from  which  the  patients  did  not  recover. 

injury  may  be  avoided 

Plant  injury  from  heat  treatment 
ranges  from  induced  dormancy  of  vege- 
tative parts  (for  example,  gladiolus  cor- 
mels),  through  delayed  seed  germina- 
tion, production  of  weak  and  deformed 
seedlings,  to  killing  of  some  or  all  of 
the  material  treated  (reduced  stand). 
Injury  may  be  reduced  by  using  seed 
relatively  free  from  mechanical  breakage 
of  the  coat,  and  by  avoiding  seed  over 
2  years  old.  Nurserymen  frequently 
overlook  the  fact  that  seed  or  propaga- 
tive  material  is  one  of  their  smallest 
expenses,  and  that  increased  rate  of 
seeding  will  adjust  for  germination  loss. 
It  often  would  be  better  to  heat-treat  the 
stock  and  eliminate  disease,  even  though 
it  reduced  the  germination  50  per  cent, 
than  to  plant  it  untreated  and  risk  100 


per  cent  loss  from  disease.  For  a  specialist 
propagator  the  economic  limits  of  treat- 
ment injury  are  much  higher;  in  many 
cases  the  survival  of  only  a  single  plant 
from  a  given  lot  is  justified  if  it  provides 
the  start  for  a  healthy  mother  block. 
Heat  treatment  has  limited  direct  use  in 
retail  production,  but  it  is  becoming  an 
invaluable  tool  in  propagative  activities. 

Value  of  treatment 

Since  heat  treatment  is  eradicative, 
aiming  at  elimination  of  the  organism 
from  the  tissue,  it  is  used  primarily  for 
situations  where  the  pathogen  is  internal 
and  not,  therefore,  reached  by  external 
chemical  treatments.  Heat  is  effective, 
however,  against  spores  and  mycelium 
both  on  the  surface  and  borne  internally. 
The  principal  value  of  heat  treatment  lies 
in  preventing  contamination  of  the  soil 
and  the  surrounding  plants  when  set  in 
the  field.  It  does  not,  however,  provide 
any  protection  of  the  plant  against  sub- 
sequent fungi  to  which  it  may  be  ex- 
posed. There  is  no  reason  from  a  disease 
standpoint,  therefore,  to  heat-treat  seed 
known  to  be  free  of  pathogens. 

There  are  several  stages  in  a  treatment 
program;  not  all  of  these  necessarily 
apply  to  each  type  of  material. 

Selecting  material  to  treat 

Select  for  treatment  the  cleanest  seed, 
plants,  or  bulbs  available.  This  would 
also  include  selecting  fresh  seed,  rather 
than  that  several  years  old.  In  general, 
only  the  cleanest  and  most  vigorous 
stock  available  is  worth  the  effort  the 
treatment  requires.  Heat  treatment  is  a 
method  for  obtaining  clean  stock,  not  a 
means  of  utilizing  low-grade  or  worth- 
less planting  material! 

Conditioning  the  material 

Get  the  material  into  the  best  possible 
condition  for  treatment.  With  many 
foliage  plants  this  may  require  growing 
them,  if  in  soil,  for  3  to  6  months  with- 
out    nitrogen     fertilization     and     only 


[223] 


enough  water  to  prevent  wilting.  A  faster 
system  might  be  to  wash  the  soil  from 
the  roots  and  set  the  plants  in  fairly 
coarse  sand  in  order  to  reduce  both 
nitrogen  and  water  intake.  Humidifiers 
and  shading  should  be  used  only  as 
necessary  to  prevent  burning.  Neph- 
thytis,  Dieffenbachia,  Fittonia,  and  Pel- 
lionia  benefit  greatly  from  hardening 
prior  to  treatment.  Such  hardening  of 
stock  should  be  practiced  before  treat- 
ing to  obtain  the  original  pathogen-free 
material  with  which  to  establish  a 
mother  block.  Further  propagation 
should,  of  course,  be  from  the  un- 
hardened  mother  block,  since  the  com- 
plete procedure  is  too  costly  and  slow  for 
general  propagation. 

It  has  been  found  that  gladiolus  cor- 
mels  produced  in  warm,  relatively  dry 
California  soils  are  more  tolerant  of  heat 
treatment  than  are  those  grown  in 
cooler  moister  areas. 

Calla  rhizomes  will  tolerate  122°  F 
for  3  hours2  when  in  the  desiccated 
market  condition,  whereas  freshly  dug, 
actively  growing  rhizomes  and  fleshy 
stems  may  be  killed  in  a  half  hour. 

Preparing  the  material 
for  treatment 

Preparation  for  treatment  involves 
trimming  of  dead  leaves,  roots,  and 
other  parts,  washing  to  remove  dirt,  and 
any  necessary  dividing  of  clumps.  No 
more  wounds  should  be  made  than  ab- 
solutely necessary,  as  water-soaking 
occurs  at  such  points.  Dieffenbachia 
cane  is  left  in  pieces  up  to  2  feet  long, 
and  is  divided  to  single-bud  pieces 
several  weeks  after  treatment.  In  our 
experience  plant  material  is  not  success- 
fully freed  of  a  pathogen  if  left  in  the 
soil-ball  or  pot.  The  cooling  effect  of  the 
soil  and  pot  on  the  water,  and  the  slow 
penetration  of  heat  into  the  soil,  prevent 
real  effectiveness.  Plants  should  always 
he  liare-rooted  for  treatment. 

I  apublished  data  of  S.  Wilhelm,  obtained 
in  the  Departmenl  of  Plant  Pathology,  Univer- 
-iiv  nf  California,  Loa  Angeles. 


Presoaking 

A  presoak  is  desirable  in  cool  water 
for  4  to  12  hours  (nasturtium  seed)  to 
displace  air  between  layers,  or  for  48 
hours  in  warm  water  (gladiolus  cor- 
mels)  to  displace  air  and  render  fungus 
mycelium  more  susceptible.  The  use  of  a 
presoak  should  be  considered  whenever 
plant  parts  have  loose  coverings  that  trap 
air  and  cause  material  to  float. 

Treatment  equipment 
and  temperature 

Heat  treatment  on  a  large  scale  is  best 
performed  in  a  tank  holding  at  least  100 
to  200  gallons  of  water,  because  the 
temperature  is  much  more  stable  with 
such  volume.  Equipment  of  this  type  is 
commercially  available  as  bulb-treat- 
ment tanks  (fig.  110;  see  also  Appen- 
dix), and  one  unit  combines  facilities 
for  steaming  soil  and  treating  plant  ma- 
terial (Sec.  10,  type  7).  Heat  may  be 
supplied  by  a  small  stream  of  water  from 
a  tap  20  to  30  degrees  F  higher  than  the 
treatment  temperature;  this  can  usually 
be  achieved  by  turning  up  the  thermo- 
stat on  the  water  heater.  Heat  may  also 
be  maintained  by  immersion-type  elec- 
tric heaters,  by  releasing  steam  into  the 
water,  or  by  gas  burners  under  the  bot- 
tom of  the  metal  tank.  With  a  little  prac- 
tice, the  temperature  can  be  kept  within 
a  half  degree  above  and  below  the  de- 
sired temperature  after  initial  adjust- 
ment; such  regulation  is  necessary  for 
the  best  results.  Water  circulation  in  a 
large  tank  is  best  achieved  by  a  circulat- 
ing pump  or  by  a  submerged  propeller. 
Circulation  is  essential  during  heat 
treatment  to  prevent  temperature  strati- 
fication and  to  promote  heat  penetration 
into  the  material. 

Thermometers  must  be  accurate  at  the 
range  120°  to  135°  F  (see  Appendix)  ; 
it  is  usually  desirable  to  have  two  or 
three  of  them  suspended  in  the  tank  to 
indicate  degree  of  temperature  uni- 
formity in  the  water.  It  is  a  worth-while 
precaution  to  have  a  precision-calibrated 


[  224  ] 


Fig.  110.  Commercial  hot-water  treatment  tank  for  bulbs,  ornamentals,  and  strawberries.  Tem- 
perature is  thermostatically  controlled,  and  the  water  is  agitated  by  a  propeller. 


thermometer  (accurate  in  the  above 
range;  see  Appendix) ,  against  which  the 
others  may  be  calibrated  at  the  exact 
temperature  used.  Thermometers  with 
expanded  scales  in  the  critical  treatment 
ranges  are  available  in  brass  protective 
cases  (see  Appendix).  These  provide 
precise  temperature  measurement  several 
degrees  either  side  of  a  specified  point. 
Since  they  are  reasonable  in  price,  they 
should  be  more  commonly  used. 

For  small  quantities  of  seed  or  plants 
we  have  had  uniformly  good  results  us- 
ing a  20-gallon  sink  or  tub,  maintaining 
uniform  temperatures  by  a  trickle  of  hot 
water,  and  stirring  with  a  wooden 
paddle. 


The  material  to  be  treated  may  be 
placed  in  open-weave  plastic  or  cloth 
bags,  or  in  screen  boxes  for  ready  han- 
dling. Seed  in  cloth  bags  should  be 
gently  kneaded  by  hand  to  expel  air 
bubbles;  material  in  screen  boxes  should 
be  constantly  agitated  for  the  same  pur- 
pose. 

The  treatment  should  be  accurately 
timed  so  that  exposure  to  heat  will  be 
precise. 

Cooling 

Prompt  cooling  of  the  treated  material 
is  imperative  in  order  to  increase  pre- 
cision of  timing  exposure.  This  may  be 
achieved  by  turning  a  hose  on  the  con- 


[225] 


tainer,  or  by  dipping  in  a  tank  of  cool 
tap  water,  again  kneading  for  rapid  heat 
transfer.  This  should  be  continued  until 
the  material  reaches  water  temperature. 
The  tank,  and  the  water  in  it,  must  be 
free  of  pathogens;  it  should  not  have 
been  used  for  soaking  untreated  material. 

Drying 

The  seed  or  stock  (except  foliage 
plants)  should  be  dried  as  rapidly  as 
possible,  certainly  within  a  few  hours. 
This  is  best  accomplished  by  spreading 
on  screens,  treated  cloth,  or  new  papers 
over  which  a  warm  dry  current  of  air  is 
blown,  and  which  are  raised  off  the 
ground.  Temperatures  in  this  phase 
should  not  exceed  90°  F,  so  that  drying 
is  accomplished  by  a  large  volume  of  dry 
air  rather  than  by  heat.  The  screen  may 
be  disinfested  with  a  formaldehyde  solu- 
tion (1  gal.  commercial  strength  to  18 
gal.  water),  followed  by  a  water  rinse, 
or  by  a  sodium  hypochlorite  spray  (0.4 
per  cent  available  chlorine)  as  explained 
in  Section  12. 

Storage 

Handling  and  storage  of  the  clean  ma- 
terial must  conform  to  rules  of  sanita- 
tion. Under  no  conditions  should  the 
material  be  placed  in  old  contaminated 
boxes,  trays,  or  bags,  or  handled  with 
dirty  tools;  these  implements  should  be 
sterilized  before  re-use  (sees.  3  and  12). 

Growing  in  isolation 

The  clean  material  must  be  grown 
isolated  from  the  general  propagation, 
to  minimize  opportunity  for  contamina- 
tion. 

Application  to  specific 
nursery  crops 

The  above  methods  are  perhaps  best 
presented  in  relation  to  specific  crops 
and  diseases.  Only  the  treatment  phases 
mentioned  are  needed  in  each  case. 

Aglaonema*  —  against  Rhizoctonia 
and    water    molds.    Old   cane   will   stand 

Previously  unpublished  data  of  the  authors. 


treatment   at   120°    F   for   30   minutes; 
cool;  plant. 

Aloe — against  water  mold  (Pythium 
ultimum)  root  rot.  Plants  of  various 
sizes  cleaned  of  debris,  treated  at  115° 
F  for  20  to  40  minutes  (the  larger  the 
plant  the  longer  the  time)  ;  cooled; 
planted.  Almost  completely  effective  in 
salvaging  infected  plants;  injury  mini- 
mal (fig.  111). 

Apium  (celery) — against  late  blight 
(Septoria  spp.)  in  seed.  Treat  seed  at 
120°  F  for  30  minutes;  cool;  dry;  plant. 
Treatment  of  seed  does  not  appreciably 
reduce  stand.  Healthy  plants  produced 
from  treated  seed  require  less  spraying 
in  the  field,  as  the  pathogen  builds  up 
slowly  from  field  debris. 

Begonia,  tuberous — against  root-knot 
nematode.  Tests  by  P.  A.  Miller3  indi- 
cated that  treatment  of  dormant  tubers 
at  120°  F  for  30  minutes  freed  them  of 
nematodes  without  injury.  Cool  and 
plant. 

Buxus  (boxwood) — against  root-knot 
nematode.  Tests  by  P.  A.  Miller3  indi- 
cated that  treatment  of  bare-root  plants 
at  118.4°  F  for  30  minutes  freed  them 
from  the  nematodes  without  serious  in- 
jury. Cool  and  plant. 

Caladium — against  Sclerotium  rolfsii 
tuber  rot.  Florida  studies  found  that 
treatment  of  dormant  tubers  at  122°  F 
for  30  minutes  eliminated  the  organism 
without  injury.  Cool  and  plant.  In  Cali- 
fornia, a  bacterial  soft  rot  of  the  rhi- 
zomes is  not  controlled  by  this  treatment. 

Capsicum  (pepper) — against  Rhizoc- 
tonia in  seed.  Treat  seed  at  125°  F  for 
30  minutes;  cool;  dry;  plant.  Com- 
pletely effective  with  almost  no  reduction 
of  germination. 

Dieffenbachia* — against  water  molds, 
bacterial  leaf  spot,  and  bacterial  soft  rot. 
Important  that  hardened  canes  be  used 
as  they  tolerate  the  necessary  125°  F; 
others  stand  only  120°  F;  young  leafy 
shoots  will  not  survive  treatment.  Treat 

:|  Department  of  Plant  Pathology,  University 
of  California,  Los  Angeles. 


[226] 


Z^Sl-SjiWi'eySy  ■';.''Zi''!'Xi!'f^^f. 


Fig.  111.  Control  of  Pythium  root  rot  of  Aloe  variegata  by  hot-water  treatment  at  115°F  for 
20  min.  A,  Treated  plants  above  and  checks  below,  20  days  after  planting.  B,  Treated  plant  at 
left  and  check  at  right,  100  days  after  planting,  showing  the  rapid  recovery  after  treatment 
and  planting  in  treated  soil. 


cane  in  pieces  about  2  feet  long  at  125° 
F  for  30  minutes;  cool;  hold  canes  in 
steamed  sphagnum  moss  until  roots  or 
buds  start;  cut  into  pieces  each  with  a 
single  bud,  plant  immediately  in  perlite 
and  peat;  replant  in  soil  when  top  is  well 
started.  Has  given  excellent  control.  In- 
creases percentage  of  buds  of  D.  bausei 
and  D.  picta  breaking  dormancy,  as  for 
Syngonium  podophyllum  below.  Tip 
cuttings  from  treated  plants,  grown  with- 
out overhead  sprinkling,  should  further 
insure  freedom  from  pathogens.  There  is 
some  evidence  that  soft-rot  bacteria  may 
be  spread  through  the  propagating  bed 
by  larvae  and  adults  of  fungus  flies. 
Spraying  the  soil  surface  with  dieldrin 
(wettable  powder,  1  oz.  per  7%  gal. 
water)  or  malathion  (wettable  powder, 
2  oz.  per  7^2  gal.  water)  has  given  some 
promise  in  controlling  these  insects. 

Fittonia* — against    Rhizoctonia    (fig. 
112)    and   water  molds.   Harden   plant. 

*  Previously  unpublished  data  of  the  authors. 


Clean  and  treat  whole  plant  at  124°  F 
(120°  if  plant  is  unhardened)  for  30 
minutes;  cool;  make  cuttings  and  divi- 
sions, removing  any  damaged  leaves; 
plant  at  once  in  perlite  and  peat,  where 
roots  may  form  in  less  than  a  week. 
Leaves  are  very  sensitive  to  treatment, 
but  stems  survive  and  new  shoots  de- 
velop. See  Section  16  for  the  experience 
of  one  grower  with  this  treatment. 

Gerbera — against  root-knot  nematode 
(Meloidogyne  incognita) .  Tests  by  S. 
A.  Slier4  indicated  that  treatment  of 
bare-rooted  plants  at  118°  F  for  20  min- 
utes freed  them  from  these  nematodes. 
Treated  plants  started  a  little  slowly,  but 
surpassed  the  untreated  ones  after  5 
months. 

Haworthia — methods  and  results  simi- 
lar to  those  with  Aloe. 

Lilium  (Croft) — against  Rhizoctonia, 
stem  and  bulb  nematode,  and  root-lesion 


4  Department  of  Plant  Nematology,  Univer- 
sity of  California,  Riverside. 


[227] 


Fig.  112.  Stem  cuttings  of  Fittonia  verschaffeltii  var.  argyroneura  in  a  treated  rooting  medium 
in  a  propagation  case,  showing  effectiveness  of  control  of  Rhizoctonia  by  hot-water  treatment. 
Above,  cuttings  treated  at  120°F  for  30  minutes;  note  the  perfect  stand  and  absence  of  leaf 
decay.  Below,  untreated  cuttings  from  the  same  lot,  showing  leaf  decay  and  poor  stand. 


[228] 


nematode;  ineffective  against  Fusarium 
basal  rot.  Tests  by  J.  G.  Bald  and  P.  A. 
Chandler5  showed  that  the  following 
schedule  was  successful  for  obtaining 
useful  commercial  stock:  Cure  bulbs  at 
95°  F  and  95  per  cent  humidity  for  1  to 
2  weeks;  presoak  2  days  in  cool  water. 
Treat  for  2  hours  in  water  plus  formalde- 
hyde (1  gal.  37  per  cent  commercial  for- 
maldehyde per  200  gal.  water)  at  115°; 
cool.  Aftersoak  the  bulbs  in  Puratized 
Agricultural  Spray  (1  pint  per  125  gal. 
water)  for  24  hours.  Scale  the  bulbs  and 
dust  scales  with  ferbam;  place  on  moist 
vermiculite  at  75°  and  95  per  cent  hu- 
midity. Remove  bulbils  and  plant  in 
treated  soil. 

Matthiola  (stock) — Seedsmen  now 
treat  seed  they  plant  so  as  to  produce 
seed  free  from  bacterial  blight.  Seed  is 
treated  by  the  producer  in  plastic  screen 
bags  at  130°  to  131°  F  for  10  minutes; 
cooled;  and  dried.  Special  techniques 
are  necessary  in  handling  because  of  the 
mucilaginous  seed  coat.  Most  stock  seed 
is  now  free  of  bacterial  blight  owing  to 
the  success  of  the  treatment. 

Pellionia* — methods  and  results  simi- 
lar to  those  with  Fittonia.  Plants  tolerate 
treatment  well,  develop  strongly. 

Philodendron  pertusum  and  P.  corda- 
tum* — against  bacterial  stem  rot  of  the 
former,  and  Rhizoctonia  on  the  latter 
(fig.  114).  No  data  available  on  effect  of 
hardening,  but  both  plants  take  120°  F 
for  30  minutes  well.  Discarded,  hard- 
ened, conservatory  plants  seem  to  take 
the  treatment  very  well.  Treat;  cool; 
root  in  steamed  sphagnum  (as  with  Dief- 
fenbachia)  ;  cut  into  propagating  sec- 
tions of  1  to  2  buds. 

Rosa — against  dagger  (Xiphinema) 
and  spiral  (Helichotylenchus)  nema- 
todes in  Florida;  wash  roots;  treat  at 
121°  F  for  3%  minutes;  cool;  dry.  If 
root-knot  or  root-lesion   (Pratylenchus) 

5  Department  of  Plant  Pathology,  University 
of  California,  Los  Angeles. 

*  Previously  unpublished  data  of  the  authors. 


nematodes  are  present,  121°  to  123°  for 
10  minutes  is  required. 

Strelitzia — against  root-rot  complex 
involving  internally  borne  Fusarium 
moniliforme.  Presoak  seed  1  day  in  water 
at  room  temperature;  treat  at  135°  F  for 
30  minutes;  cool;  dry;  plant.  Commer- 
cially effective  in  preventing  loss  of  seed- 
lings without  reducing  germination. 

Syngonium  auritum  (Philodendron 
trifoliatum) — against  black  cane  rot 
(caused  by  a  specialized  form  of  the 
fungus  Ceratocystis  fimbriata) .  Harden- 
ing increases  heat  tolerance  of  plants. 
Treat  whole  bare-rooted  plants  at  120° 
F  for  30  minutes;  cool;  plant  in  soil. 
Eliminates  pathogen;  some  leaf  injury, 
but  plants  quickly  recover. 

Syngonium  podophyllum* — canes  are 
treated  in  pieces  about  2  feet  long ;  toler- 
ate treatment  without  injury.  Clean  and 
treat  at  120°  F  for  30  minutes;  cool; 
handle  as  for  Fittonia.  This  treatment 
breaks  dormancy  of  the  cane  buds, 
greatly  increasing  the  rapidity  and  suc- 
cess of  propagation  (fig.  113).  Present 
procedure  is  to  hot-water-treat  as  above, 
then  hold  in  a  humidity  cabinet  at  70°  F 
for  2  to  3  weeks  before  planting. 

Weigelia — against  root-knot  nema- 
tode. Tests  by  P.  A.  Miller6  indicate 
that  treatment  of  bare-root  plants  at 
120°  F  for  30  minutes  freed  them  of 
nematodes  without  serious  injury.  Cool 
and  plant. 

Zantedeschia  aethiopica  (white  calla) 
— against  water  mold  {Phytophthora 
richardiae) .  Clean  dormant  rhizomes; 
treat  at  122°  F  for  1  hour;  cool;  dry. 

Zinnia — against  Alter naria  disease 
(A.  zinniae)  and  Rhizoctonia.  Seed 
should  not  be  more  than  1  year  old,  or 
germination  will  be  reduced  by  treat- 
ment. Treat  seed  at  125°  F  for  30  min- 
utes; cool;  dry.  Commercially  effective 
in  eliminating  these  pathogens  from 
fresh  seed,  without  serious  injury. 

6  Department  of  Plant  Pathology,  University 
of  California,  Los  Angeles. 


[229] 


Fig.  113.  Effect  on  growth  of  Syngonium  podophyllum  (Emerald  Gem)  stem  cuttings  of  hot- 
water  treatment  (120°F  for  30  min.)  and  holding  under  humid  conditions  and  70°  for  2  weeks. 
Photos,  after  7  weeks'  growth  in  the  glasshouse,  show  stimulation  of  bud  and  root  development 
due  to  breaking  of  dormancy.  Upper  left,  untreated  check,  planted  at  once  in  the  glasshouse. 
Upper  right,  untreated  check,  held  at  70°.  Lower  left,  treated  and  planted  at  once  in  the  glass- 
house. Lower  right,  treated  and  held  at  70°. 


Chemical  Treatment  of 
Planting  Stock 

This  method  is  effective  only  against 
pathogens  externally  carried  on  the 
planting  material  where  the  chemical 
will  come  in  contact  with  them.  It  is, 
therefore,  of  limited  value  to  nursery- 
men. 

Nurserymen  should  realize  that  chemi- 
cal seed  treatments  are  of  two  types 
(protective  and  eradicative),  only  one  of 
which  aims  at  elimination  of  disease 
organisms  from  the  seed.  It  is  a  potenti- 
ally disastrous  misconception  that  the 
"treated"  seed  sold  in  packets  today  is 
free  of  pathogens.  The  seed  is  treated 
with  a  protective  mild  fungicide  (for  ex- 
ample Spergon  on  sweet  pea  seed)  to 
protect  it  from  decay  prior  to  emergence, 


or  to  reduce  damping-off  in  slightly  in- 
fested soil.  Treatments  that  eradicate 
pathogens  from  seed  usually  reduce 
germination,  and  therefore  are  not  ap- 
plied before  sale.  Three  examples  of 
eradicative  chemical  treatment  may  be 
cited. 

1.  Dormant  white  calla  rhizomes  are 
soaked  for  1  hour  in  formaldehyde  (1 
pint  to  6%  gal.  water),  mercuric  chlo- 
ride (one  7^/2  grain  tablet  per  pint  of 
water),  or  in  a  suspension  of  8  ounces 
New  Improved  Ceresan  plus  1  ounce 
Dreft  per  25  gallons  of  water.  This  will 
free  them  of  the  root  rot  Phytophthora. 
Since  growth  may  be  somewhat  delayed 
by  these  treatments,  the  heat  method  de- 
described  above  may  be  preferable. 

2.  Seed  of  China  aster  may  be  treated 


|  230  ] 


Fig.  114.  Control  of  Rhizoctonia  root  rot  of  Philodendron  cordatum  plants  grown  on  poles  in 
pots.  Plants  shown  above  are  in  the  soil  ball;  the  same  plants  are  shown  below  with  roots  washed 
free  of  soil.  Plants  at  left  as  they  were  obtained  from  a  commercial  nursery,  showing  severe  root 
decay;  comparable  plants  at  right  3  months  after  the  root  systems  and  the  poles  were  washed 
free  of  soil,  treated  in  hot  water  (120°F  for  30  min.)  and  replanted  in  treated  soil. 


[231] 


with  an  unheated  mercuric  chloride  solu- 
tion (one  l1/-!  grain  tablet  per  pint  of 
water)  to  free  it  of  the  wilt  Fusarium.  A 
given  volume  of  seed  is  placed  in  a  jar, 
covered  for  30  minutes  with  three  times 
that  volume  of  mercuric  chloride  and 
intermittently  shaken.  The  solution  is 
poured  off,  the  seed  rinsed  with  several 
changes  of  water,  and  dried.  Some  germi- 
nation reduction  may  result,  particularly 
in  seed  with  cracked  coats. 

3.  The  foliar  nematode  (Aphelen- 
choides  ritzema-bosi)  of  chrysanthemum 
may  be  eliminated  from  a  cutting  bed 
by  chemical  treatment.  A  spray  of  para- 
thion  (25  per  cent  wettable  powder,  1 
lb.  per  100  gal.  water,  plus  4  to  6  fl.  oz. 
Triton  B-1956)  will  be  absorbed  by  the 
plant  and  poison  the  nemas.  Formerly 
sodium  selenate  applications  to  the  soil 
were  used  for  the  same  purpose.  Sodium 
selenate  or  Demeton  (Systox)  are  used 
to  free  Saintpaulia  of  foliar  nematode. 

Sanitary  Measures 

It  is  rarely  possible  to  free  stock  of  a 
pathogen  by  removal  of  diseased  tissue. 
However,  azalea  plants  may  be  freed  of 
the  flower-blight  fungus,  Ovulinia  aza- 
leae,  by  carefully  removing  all  dead 
flowers  from  the  plant,  and  stripping  off 
the  top  inch  of  rooting  medium,  or  by 
bare-rooting  the  plant.  Thus,  carryover 
sclerotia  are  eliminated,  breaking  the 
pathogen's  cycle. 

Similarly,  the  flower-blight  fungus 
(Sclerotinia  camelliae)  of  camellia  may 
be  eliminated  by  careful  removal  and 
burial  or  burning  of  flowers.  If  no 
flowers  are  permitted  on  young  nursery 
stock,  and  not  allowed  to  fall  into  the 
cans  from  large  flowering  plants,  there 
can  be  no  carryover  of  the  fungus  with 
the  plants.  Removal  of  the  top  inch  of 
soil,  as  for  azalea,  will  be  similarly  ef- 
fective. 

Aging  of  Seed 

Rarely  the  fungus  will  survive  for  a 
shorter  period  of  time  than  will  the 
plant  seed    in    which   it  developed.   It  is 


possible  to  free  celery  seed  of  the  late- 
blight  Septoria  by  holding  it  for  3  years 
or  more  before  planting.  This  procedure 
is  applicable  only  on  celery  seed  among 
the  nursery  crops. 

Prolonged  Roguing  of  Diseased 
Plants  from  a  Stock 

Diseases  have  periods  when  they  may 
not  be  detectable  in  plants  that  later  may 
show  striking  symptoms.  This  makes  it 
impossible  to  eliminate  infected  plants 
by  a  single  roguing.  If,  however,  the 
causal  agent  has  essentially  no  natural 
spread,  it  is  possible  by  careful  and  re- 
peated roguing  over  a  period  of  several 
years  to  clean  up  a  stock  and  thus  estab- 
lish a  healthy  mother  block.  This  is  true 
of  many  of  the  viruses  of  woody  plants, 
as  for  example,  rose  mosaic. 

If  the  virus  spread  occurs  only  with 
budwood  or  scions  it  is  possible  to  index 
plants  and  issue  registered  budwood 
from  disease-free  specimens.  This  is 
being  done  for  citrus  stock  free  of  psoro- 
sis  virus,  and  for  some  of  the  stone 
fruits. 

Virus-indexing  Methods 

Recognition  of  virus  infection  is  diffi- 
cult in  plants  which,  for  various  reasons, 
temporarily  or  permanently  fail  to  show 
symptoms.  To  find  virus-free  plants 
under  these  conditions,  special  indexing 
procedures  are  required.  Such  methods 
are  highly  technical,  but  are  already  in 
commercial  use  by  specialist  propa- 
gators. Brierley  and  Olson  (1956)  have 
described  a  method  for  graft-indexing 
chrysanthemums  to  highly  susceptible 
healthy  varieties  in  order  to  detect  virus- 
infected  plants.  Only  the  virus-free  plants 
are  then  propagated.  Gasiorkiewicz  and 
Olson  (1956)  have  described  a  similar 
method  for  carnations.  The  presence  of 
mechanically  transmissible  viruses  of 
either  plant  may  be  detected  by  sap 
transfer,  usually  by  the  use  of  carbo- 
rundum powder,  to  the  leaves  of  some 
host  which  shows  symptoms.  With  some 


[232] 


viruses  (aster  yellows)  the  use  of  an  ap- 
propriate insect  vector  is  necessary. 

Growing  Plants  from  Seed 

Disease  agents  frequently  are  not  car- 
ried through  the  seed  but  rapidly  build 
up  through  vegetative  propagation.  This 
is  particularly  true  of  viruses.  Thus, 
raising  ranunculus,  anemone,  and  free- 
sias  from  claws,  roots,  or  corms  leads  to 
serious  losses  from  mosaic,  whereas  the 
loss  is  minimal  when  grown  from  seed. 
Similarly,  yellow  calla  grown  from  true 
seed  is  free  of  bacterial  soft  rot,  water 
molds,  and  Rhizoctonia,  all  of  which  are 
commonly  carried  on  the  rhizomes.  Ap- 
parently it  is  possible  to  obtain  ferns 
free  of  foliar  nematode  (Aphelenchoides 
fragariae)  by  starting  with  spores  from 
leaves  kept  dry  during  growth. 

Selecting  the  Growing  Region 

The  semiarid  California  climate  is  an 
effective  ally  in  eliminating  many  nurs- 
ery diseases  because  plants  dry  off 
quickly  after  rain  or  overhead  watering. 
Many  fungi  require  prolonged  moist 
conditions  in  order  to  produce  spores, 
which  may  in  turn  be  spread  only  in 
water  and  must  be  wetted  for  a  few  to 
48  hours  to  infect  the  host. 

Many  diseases,  important  elsewhere, 
are  essentially  unknown  here  because  of 
the  climate,  among  them  snapdragon 
anthracnose  and  Phyllosticta  leaf  spot, 
Phomopsis  canker  of  aster,  bacterial  leaf 
spot  of  poppy,  and  Ascochyta  flower 
blight  of  chrysanthemum.  Septoria  leaf 
spot  of  chrysanthemum  is  important 
only  in  propagating  frames  or  plantings 
extensively  watered  from  above.  Seed  of 
China  aster  free  of  Stemphylium  calli- 
stephi,  and  zinnia  free  of  Alter naria  zin- 
niae  may  be  produced  in  California  by 
growing  in  dry  inland,  instead  of  coastal, 
valleys.  Azalea  and  camellia  flower 
blights  largely  occur  in  California 
under  lath  or  shrubbery  cover.  Promot- 
ing air  movement  through  the  lath  house 


by  removal  of  the  sides  will  reduce  the 
disease  by  restoring  conditions  of  rapid 
drying. 

Although  there  are  few  nursery  dis- 
eases that  can  be  eliminated  solely 
through  selection  of  the  region,  growers 
should  exploit  this  climatic  advantage  of 
the  state  to  the  maximum  in  support  of 
other  controls. 

Seed  or  plants  grown  under  dry  condi- 
tions without  overhead  watering  often 
are  freest  from  disease.  They  should  be 
grown-on  without  sprinkling  or  syring- 
ing whenever  possible. 

Reporting  Diseased  Stock  to 
the  Propagator 

This  point  is  important  because  it  is 
sometimes  the  primary  force  in  bringing 
about  improvement.  Propagators  may 
actually  be  unaware  of  the  situation  and 
may  be  grateful  for  notification  of  it,  or 
they  may  mistakenly  think  that  since 
perfection  is  unattainable,  a  fairly  clean 
stock  is  good  enough.  They  can  no  more 
have  "a  little  disease"  than  they  can  be 
a  little  dead!  They  will,  in  any  case,  cer- 
tainly respond  faster  to  customer  pro- 
tests than  to  advice  from  Agricultural 
Experiment  Station  workers  or  state 
nursery  inspectors.  In  this  democratic 
process  of  the  business  world,  the  grower 
who  accepts  diseased  stock  without  pro- 
test is  derelict  of  his  duty  in  the  system. 
Since  in  the  long  run  results  may  be 
achieved  in  this  way  that  apparently  can- 
not otherwise  be  accomplished,  there  is 
little  virtue  in  being  a  silent  sufferer. 

Of  the  rare  propagator  who  inten- 
tionally markets  diseased  stock  little 
need  be  said,  since  he  is  self-exterminat- 
ing. If  he  clips  the  scattered  galls  of  root- 
knot  nematode  from  his  rose  roots  or 
cuts  out  the  swellings  of  bacterial  fascia- 
tion  from  his  divisions  of  Esther  Read 
daisies  before  shipment,  he  is  practicing 
fraud  rather  than  disease  control,  and 
this  will  eventually  be  recognized  by  his 
customers. 


[233] 


MAINTAINING  CLEAN  STOCK 


Once  healthy  nursery  stock  is  ob- 
tained, one  faces  the  problem  of  main- 
taining it  in  this  condition.  In  actual 
practice,  clean  stock  is  fairly  commonly 
obtained  but  is  so  quickly  infected  that 
the  grower  does  not  become  aware  of  its 
value.  New  seedling  varieties  are  almost 
always  free  of  virus  diseases  as  well  as 
most  pathogens,  and  in  this  sense  plant 
breeding  is  a  primary  source  of  healthy 
stock.  Usually  no  effort  has  been  ex- 
pended to  maintain  the  variety  in  this 
condition,  and  it  is  soon  discarded  be- 
cause of  disease;  the  rapid  commercial 
disappearance  of  the  King  Cardinal 
carnation  because  of  mosaic  is  but  one 
of  many  instances  of  this  sort.  This  loss 
more  often  results  from  lack  of  knowl- 
edge concerning  the  desirability  of,  and 
methods  for,  maintaining  the  healthy 
condition  of  the  plants  than  from  in- 
surmountable difficulties  in  the  system. 
For  example,  a  large  eastern  propagator 
is  developing  and  introducing  new  seed- 
ling varieties  of  carnations  that  are  free 
from  most  of  the  major  diseases.  They 
are  maintaining  stock  of  these  varieties 
under  conditions  that  largely  prevent 
their  becoming  diseased.  Growers  buy- 
ing such  a  variety  sometimes  find  that 
it  develops  "excessive  growth"  the  first 
year  (in  large  part  because  of  freedom 
from  disease) ,  and  usually  note  that  pro- 
duction drops  in  a  few  years  under  their 
own  propagation.  They  then  buy  new 
cuttings  of  the  variety  and  start  over. 
This  company  keeps  the  variety  disease- 
free,  rather  than  permitting  it  to  become 
infected  (as  the  grower  and  the  propa- 
gator have  done  in  the  past),  so  that 
there  is  now  a  clean  stock  to  fall  back  on. 
The  more  important  basic  procedures 
for  preventing  contamination  of  stock 
are  given  below. 


Isolating  New  Stock 
Almost  every  nurseryman  has  known 


an  instance  in  which  a  small  quantity  of 
new  stock  was  purchased  and  set  among 
healthy  local  plants,  introducing  disease 
to  the  whole  lot.  A  few  progressive 
growers  have  an  "isolation  ward"  or 
greenhouse  to  which  new  stock  is  sent 
until  freedom  from  disease  is  established. 
Such  a  practice  has  the  full  weight  of 
scientific  theory  and  bitter  experience 
behind  it.  The  marvel  is  that  it  is  not 
more  frequently  used. 

This  process  is  used  in  addition  to  any 
inspection  and  certification  practices. 
Many  diseases  are  not  detectable  at  first 
by  inspection  because  there  is  always  a 
lag  (called  the  incubation  period)  be- 
tween infection  and  symptom  appear- 
ance. Snapdragon  rust,  for  example, 
shows  no  symptoms  for  about  a  week 
and  is  not  conspicuous  for  10  to  14 
days.  Other  diseases  may  appear  only  at 
certain  stages  of  growth,  as  for  example, 
azalea  and  camellia  flower  blights  dur- 
ing blooming.  Still  other  diseases  (for 
example,  geranium  mosaic)  may  be 
evident  for  only  brief  periods  and  then 
disappear.  The  safe  procedure  is,  there- 
fore, to  isolate  introduced  plants  until 
their  health  is  established.  Because  of  the 
complexities  exemplified  above,  the 
grower  should  consult  the  local  farm  ad- 
visor or  the  Agricultural  Experiment 
Station  in  arranging  a  program  of  this 
sort.  Another  reason  for  using  pathogen- 
free  stock  is  that  it  may  be  brought  into 
a  nursery  without  preliminary  isolation. 

The  use  of  a  "pest  house"  is  highly 
desirable  in  most  nurseries,  and  is  abso- 
lutely required  in  one  that  is  raising 
pathogen-free  stock  for  sale.  Because 
some  pathogens  or  viruses  are  carried 
by  wind  and  insects,  it  is  essential  that 
the  isolation  house  really  be  separated 
from  the  main  plantings. 

The  same  principle  may  be  involved 
in  the  handling  of  single  plants.  Thus, 
callas    are    commonly    grown     in     pots 


I  234  ] 


rather  than  benches  in  order  to  prevent 
the  spread  of  Phytophthora  root  rot  and 
bacterial  soft  rot. 

Isolating  Propagative  Operations 

At  first  seedlings  are  quite  free  of 
virus  infection,  gradually  accumulating 
viruses  with  time.  It  is  important  to  de- 
lay such  infection  by  not  locating  a  seed 
or  cutting  bed  near  any  source  of  infec- 
tive insects. 

Thus,  raising  delphinium  seedlings 
near  an  old  planting  of  delphinium  or  a 
growing  celerv  field  is  an  invitation  to 
leafhoppers.  They  carry  the  aster-yellows 
virus  and.  in  feeding,  infect  the  seed- 
lings; symptoms  may  not  appear,  how- 
ever, until  the  plants  are  growing  in 
home  yards.  Such  seedbeds  might  well  be 
tightly  covered  with  aster  cloth  or  plastic 
screens. 

Seedlings  of  aster,  tomato,  and  pepper 
raav  be  infected  with  the  spotted-wilt 
virus  through  migration  of  thrips  from 
surrounding  infected  plants.  In  one  in- 
stance tomato  seedlings  were  thus  in- 
fected from  old  diseased  dahlia  plants 
grown  bv  the  nurservman's  wife  for 
cut  flowers  just  outside  the  glasshouse 
ventilators.  Certain  coastal  areas  have 
severe  outbreaks  of  spotted  wilt  each 
vear.  These  "endemic  areas"  are  located 
in  the  cool  coastal  fog  belt,  and  have  a 
persistent  population  of  dahlias,  nas- 
turtiums, callas,  buttonweed,  chickweed, 
and  other  hosts.  Such  endemic  areas 
should  be  avoided  for  propagation  of 
certain  plants. 

Growing  stock  seedlings  in  an  area 
surrounded  by  wild  radish  and  mustard 
is  another  dangerous  situation.  Aphids 
carry  mosaic  to  the  stocks,  which  may 
not  show  symptoms  until  flowering. 

In  general,  propagative  activities 
should  never  be  conducted  in  a  weedy- 
area  or  in  one  with  near-by  fields  of 
virus-infected  herbaceous  crops.  Because 
of  the  complexities  involved,  consult 
your  farm  advisor  for  details  in  specific 
cases. 


The  Mother-Block  Principle 

When  one  has  purchased  or  developed 
healthy  stock,  it  should  be  grown  in 
isolation  and  with  special  care.  To  main- 
tain this  material  in  a  separate  planting 
is  the  mother-block  principle.  It  is  ob- 
viously easier  to  prevent  introduction  of 
a  pathogen  into  a  small  special  block  of 
stock  than  into  large  plantings.  It  is 
easier  to  isolate  such  a  small  nucleus 
planting,  to  inspect  it  carefully  and  re- 
move any  diseased  plants,  and  to  control 
insects  that  may  spread  virus  diseases. 
The  job  is,  therefore,  more  likely  to  get 
done. 

There  should  be  only  one-way  traffic 
with  the  established  mother  block;  cut- 
tings or  plants  may  be  taken  out  but  no 
plants  should  be  brought  into  it  (fig. 
115).  Above  all.  no  buds  or  grafts 
should  be  placed  in  plants  of  the  block; 
undetected  viruses  and  V erticillium  wilt 
can  thus  be  introduced  and  ruin  the 
planting.  Visitors  should  not  be  per- 
mitted in  the  glasshouses  containing  the 
mother  blocks. 

By  carefully  selecting  within  mother- 
block  plants,  one  can  prevent  both  loss 
of  horticultural  quality  and  increased 
variation,  and  may  thus  be  able  to  effect 
distinct  improvements.  The  maintenance 
of  the  mother  block,  and  the  selection  of 
plants  in  it,  should  be  under  the  personal 
supervision  of  the  owner,  never  referred 
to  routine  help.  Growers  who  do  not 
maintain  mother  blocks  often  sell  the 
best  stock  in  an  effort  to  please  their 
customers,  and  then  plant  the  remainder 
for  next  year's  propagation.  Because 
many  diseases  cause  decreased  plant  size 
and  vigor,  such  a  practice  may  actually 
be  selecting  toward  disease.  The  phe- 
nomenon of  "running  out"  of  horticul- 
tural varieties  probably  largely  repre- 
sents such  accumulation  of  disease  and 
weak  variants.  The  production  and  mer- 
chandising of  plants,  and  the  mainte- 
nance of  the  basic  stock  for  future  propa- 
gation, must  be  handled  as  independent 


[  235  ] 


'  '*, 


a 


Pathogen-free  stock  obtained  from: 
Chance  healthy  plants 
New  seedlings 
Practices  which  enable  plant 

to  grow  away  from  pathogen 
Cultured-cutting  technique 
Heat  treatment 
Chemical  treatment 
Sanitary  practices 
Selected  growing  areas  or  conditions 


>F 


Propagate 
and  grow 


Careful  check 
for  freedom 
from  pathogens 


Establish  isolated 
permanent  mother 
blocks 


Healthy 
— ^     propagating 
stock  sold 


a 
o 


3 
TJ 
O 


E 
E 
o 
u 


Propagate, 

plant, 
and  grow 


Crop      "*  Propagate, 

pro-       ^ —  plant, 

duction  and  grow 


Grow  on 
V_     for  crop  ^_  __  _ 


Check  for 

freedom  from 

pathogens 

before  each 

propagation 


Propagate 
from  plants     z. 

if  healthy 
+ 


Grow  a  single  crop,  and 
discard  plants. 


Grow  in  isolation  for  single 
early  propagation. 


Plant  to  establish  temporary 

mother  block.  Must  be 

isolated  from  commercial 

production. 


Grow  for  crop 


Fig.  115.  Diagram  showing  the  segregation  of  propagation  and  commercial  production,  and 
the  sequences  of  operations  that  may  be  followed.  Only  in  the  last  method  (not  recommended; 
used  only  in  exceptional  cases)  is  propagating  material  taken  from  commercial  production  stock. 
Commercial  growing  activities  shown  in  boldface  type,  stock  propagation  in  ordinary  type. 
Treated  soil  and  containers  to  be  used  throughout.  See  text  for  details  and  specific  crops. 


and  isolated  activities.  To  do  so  is  a 
long-term  service  to  the  nursery  indus- 
try, the  grower,  and  his  customers. 

An  example  of  the  danger  of  not  hav- 
ing a  mother  block  of  rootstock  may  be 
cited.  A  nursery  budded  one  of  its  new 
hybrid  rose  seedlings  on  three  under- 
stocks taken  from  a  commercial  field 
and,  after  the  plants  were  established, 
the  seedling  was  lost.  Buds  taken  from 
these  three  plants  were  placed  in  a  field 
increase-block.  A  large  percentage  of  the 
plants  developed  variegated  symptoms 
of  yellow  mosaic,  and  it  was  found  that 
two  of  the  three  original  plants  had  been 
infected    from    the    rootstocks.    It    was 


CAUTION:  Many  of  the  chemicals 
mentioned  in  this  manual  are  poi- 
sonous and  may  be  harmful.  The 
user  should  carefully  follow  the  pre- 
cautions on  the  labels  of  the  con- 
tainers. 


necessary  to  destroy  all  detectably  in- 
fected plants  and  delay  introduction  for 
another  year.  The  world  thus  narrowly 
missed  losing  what  proved  to  be  a  fine 
Ail-American  rose. 

Sanitary  Procedures 

This  principle  for  maintaining  healthy 
stock  is  less  specific  than  the  foregoing, 
and  therefore  more  difficult  to  apply.  It 
is,  however,  of  the  utmost  importance 
that  careful  sanitation  be  practiced.  A 
number  of  procedures  have  been  out- 
lined for  this,  and  are  summarized  in 
"A  Nursery  Sanitation  Code,"  Section  1. 

The  essence  of  a  successful  sanitation 
program  is  the  positive  mental  attitude 
of  the  grower.  One  should  analyze  the 
many  nursery  operations  for  potential 
"leaks"  of  contamination  and  eliminate 
them  before  trouble  appears,  rather  than 
wait  until  the  appearance  of  disease 
forces  corrective  measures.  This  manual 
provides  information  necessary  for  such 
analysis. 


[  236  ] 


SECTION 


0  m 
i 


Beneficial  Soil 
Microorganisms 


John  Ferguson 


Microorganisms  in  the  soil 

Transformation  of  plant  nutrients  by  microorganisms 
Effect  of  soil  treatments  on  microorganisms 
Controlled  colonization:  a  future  step 


Xhis  section  deals  with  the  numerous 
biological,  physical,  and  chemical  proc- 
esses in  the  soil  which  result  from  the 
activities  of  microorganisms.  An  at- 
tempt is  made  to  portray  briefly  the  soil 
microbiological  population,  the  interre- 
lations among  microorganisms,  the 
microbial  transformation  of  various  ele- 
ments essential  for  plant  growth,  the  ef- 
fect on  plant  disease  and  nutrition  of 
man's  activities  in  modifying  the  soil 
population,  and  the  general  applications 
of  this  knowledge  to  the  growing  of 
better  plants. 

One  of  the  essential  functions  of  soil 
organisms  is  decomposing  organic  mat- 


ter and  converting  to  forms  available  to 
crop  plants  such  elements  as  carbon, 
nitrogen,  calcium,  magnesium,  potas- 
sium, phosphorus,  sulfur,  iron,  and  zinc. 
There  are  only  limited  sources  of  several 
of  these  elements,  especially  carbon, 
nitrogen,  and  phosphorus,  in  a  form 
available  to  plants.  These  essential  ele- 
ments must  be  returned  to  inorganic, 
available  forms,  a  process  carried  out 
primarily  by  the  soil  population.  Thus, 
through  their  various  activities,  soil 
microorganisms  enable  life  to  continue 
by  keeping  in  constant  circulation  the 
elements  most  essential  for  plant  and 
animal  life. 


MICROORGANISMS  IN  THE  SOIL 

Abundance  terial  in  normal  field  soil  there  are  about 

Untreated  natural  top  soil  has  a  vast  10  pounds  of  microorganisms.  Soil  fungi 

population    of    microscopic    plants    and  alone    consistently    occur    in    quantities 

animals.  The  members  of  this  extremely  weighing    1,700   pounds   per   acre,    and 

active  population  include  many  forms  of  they  are  only  one  of  the  numerous  repre- 

life.  For  every  100  pounds  of  plant  ma-  sentatives  of  the  soil  community.   Bac- 

[237] 


teria  commonly  occur  in  amounts  of  30      tain  plant  parasites  may  persist  in  soil 
to  40  pounds  per  acre,  each  pound  com-      for  several  months,  however,  and  some 


prising  upwards  of  500,000,000,000  in- 
dividual bacteria.  (Five  hundred  to  1,000 
bacteria  placed  in  a  line  would  extend 
across  the  head  of  a  pin.)  It  is  interesting 
that  another  group  of  microbes  very 
similar  to  bacteria,  the  actinomycetes, 
occur  in  high  enough  concentrations  to 
give  soils  their  typical  musty  odor.  Most 
of  this  vast  microbial  population  is  con- 
centrated in  the  upper  1  to  3  feet  of  soil, 
the  depth  depending  upon  the  environ- 
ment and  nutrient  availability. 

Competition 

Soil  may  well  be  likened  to  a  minia- 
ture jungle  in  which  some  species  prey 
upon  others  and  all  compete  for  avail- 
able space  and  nutrients.  All  forms  of 
microscopic  life  (including  bacteria, 
fungi,  actinomycetes,  algae,  nematodes, 
and  protozoa)  exist  in  the  soil  in  a  state 
of  dynamic  equilibrium,  or  ever-chang- 
ing balance.  Any  change,  such  as  in  food 
supply  or  environment,  affects  all  mem- 
bers of  the  colony  and,  therefore,  dif- 
ferent species  become  dominant  from 
time  to  time.  Any  new  addition  to  this 
population  faces  intense  competition  and 
often  perishes  or  develops  slowly  until 
some  factor  shifts  to  its  advantage.  Thus 
for  any  so-called  "microbial  inoculant" 
to  be  effective,  the  environment  must  be 
favorable  and  a  ready  nutrient  source 
must  be  available. 

Relation  to  Crop  Plants 


Parasites  and  saprophytes 

In  their  relation  to  plants,  micro- 
organisms can  be  considered  as  either 
parasites  or  saprophytes.  The  parasitic 
forms  are  capable  of  growing  on  the  liv- 
ing plant  and  causing  its  decreased 
growth  or  death.  Saprophytic  microor- 
ganisms grow  only  on  dead  tissue  or  its 
decomposition  products.  The  typical  soil 
population  consists  of  organisms  which 
arc  largely  saprophytic  in  nature.  Cer- 

[238] 


are  capable  of  leading  a  normal  exist- 
ence there.  In  water  molds,  for  example, 
the  parasitic  activity  may  be  relatively 
unimportant  for  the  survival  of  the  or- 
ganism, and  only  incidental  to  its  normal 
saprophytic  life. 

Within  one  species  of  the  fungus 
genus  Fusarium  are  purely  saprophytic 
forms  and  highly  specialized  vascular 
parasites  capable  of  saprophytic  sur- 
vival. This  illustrates  the  escape  from 
competition  achieved  through  the  spe- 
cialization of  parasitism.  As  a  sapro- 
phyte an  organism  must  compete  with 
the  majority  of  the  soil  population  for 
the  available  organic  matter  and  mineral 
elements,  while  as  a  parasite  capable  of 
penetrating  a  living  plant,  most  of  the 
competition  is  eliminated.  The  parasite 
damages  the  plant  not  only  through  its 
own  activities,  but  also  by  providing  an 
entrance  for  secondary  organisms  ca- 
pable of  damaging  the  plant  but  unable 
to  penetrate  alone. 

Direct  and  indirect  effects 

The  number,  kinds,  distribution,  and 
interrelations  of  soil  organisms  have  a 
very  decided  effect  on  plant  growth. 
Within  this  delicately  balanced  complex 
are  organisms  favorable  to  plant  growth 
and  those  which  affect  growth  adversely. 
Included  in  the  group  which  directly 
favor  plant  growth  ("beneficial,"  fig. 
116)  are,  for  example,  microbes  which 
aid  in  making  essential  nutrients  avail- 
able, decompose  toxic  materials,  or  im- 
prove soil  structure.  Organisms  which 
inhibit  plant  growth  ("harmful,"  fig. 
116)  may  do  so  either  by  direct  attack, 
as  with  parasites  or  predators,  by  com- 
petition for  space  or  nutrients,  or  by  the 
release  of  toxic  substances. 

As  illustrated  in  figure  116,  the  harm- 
ful or  the  beneficial  organisms  are  in 
turn  affected,  either  favorably  or  ad- 
versely,   directly    by   other   microbes   or 


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beneficial  to  the  plant. 

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BENEFICIAL  ORGANISMS 
4 

Ex.  Ammonifying  bacteria  and 
fungi,  Nitrifying  bacteria 

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indirectly  through  a  series  of  organism 
reactions.  All  soil  microbes  thus  ulti- 
mately contribute  to  either  the  increase 
or  decrease  of  plant  growth. 

Directly  beneficial  organisms  are 
those  which  attack  organic  matter  and 
rehase  elements  in  a  form  available  for 
plants,  for  example,  ammonifying  and 
nitrifying  bacteria. 

Directly  harmful  microbes  are  repre- 
sented by  plant  pathogens  such  as 
Rhizoctonia  solani  or  V erticillium  albo- 
atrum. 

The  soil  fungus  Trichoderma,  which 
attacks  Rhizoctonia,  is  a  good  example 
of  an  indirectly  beneficial  organism.  The 
nematode-trapping  fungi  also  illustrate 
the  indirectly  beneficial  effect. 

An  organism  antagonistic  to  Tricho- 
derma would  represent  a  harmful  agent 
working  several  steps  removed  from  the 
plant  since  a  reduction  of  Trichoderma 
could  cause  Rhizoctonia  damage  to  in- 
crease and  thus  decrease  plant  growth. 
There  are  numerous  cases  where  organ- 
isms attack  other  organisms,  change  pH, 
or  tie  up  essential  elements,  and  the  re- 
sulting shift  in  the  soil  community 
markedly  affects  the  plant.  These  will  be 
illustrated  further  in  the  section,  and 
work  will  be  described  in  which  bene- 
ficial organisms  were  used  to  reduce 
disease  loss  and  to  make  nitrogen  more 
readily  available. 

Distribution  of  Microorganisms 

The  distribution  of  the  types  and 
numbers  of  organisms  comprising  the 
complex  soil  population  is  controlled  by 
the  nature  and  availability  of  nutrients, 
the  physical  and  chemical  properties  of 
the  soil,  and  the  environmental  condi- 
tions, especially  aeration,  temperature, 
and  moisture  supply.  Because  of  their 
extremely  small  size,  tremendous  abun- 
dance and  power  of  multiplication,  and 
wide  variability  in  growth  requirements, 
microbes  are  universally  distributed  in 
nature. 


Concentration  in  surface  layer 

Microorganisms  occur  in  largest  num- 
bers in  the  surface  layer  of  soil,  which 
varies  in  thickness  from  a  few  inches  to 
several  feet.  The  greatest  abundance  and 
variety  of  individuals  are  found  either 
at  the  very  surface  of  soils  (for  example, 
soils  covered  with  litter  in  forests, 
meadows,  or  other  shaded  areas)  or  just 
below  the  surface  as  in  regularly  culti- 
vated fields. 

Effect  of  organic  matter 

The  numbers  of  microorganisms  de- 
crease with  depth,  the  amount  of  decrease 
varying  with  soil  conditions,  especially 
the  distribution  of  organic  matter  and 
the  degree  of  aeration.  The  numbers, 
types,  and  activities  of  microorganisms 
fall  off  rapidly  in  shallow  soils  deficient 
in  organic  matter.  An  excellent  example 
is  the  wind-blown  soils  recommended  for 
the  U.  C.  soil  mixes.  As  few  as  17,000 
organisms  per  gram  have  been  reported 
from  such  soils  containing  about  0.3 
per  cent  organic  matter.  These  organ- 
isms were  largely  bacteria.  The  same 
type  of  soil  with  0.45  per  cent  organic 
matter  gave  an  average  of  60,000  organ- 
isms per  gram.  Common  agricultural 
soils  containing  3  to  5  per  cent  organic 
matter  have  from  1,000,000  to  500,000,- 
000  organisms  per  gram.  These  facts 
show  the  close  relation  between  organic 
matter  and  soil  microorganisms. 

Concentration  in  the 
rhizosphere 

The  rhizosphere  is  the  name  given  to 
that  area  of  soil  immediately  around 
and  on  plant  roots,  which  supports  in- 
creased microbiological  activity.  There 
are  from  three  to  fifty,  times  as  many 
organisms  in  the  rhizosphere  as  in  soil  • 
not  closely  associated  with  plant  roots. 
This  increased  activity  in  the  root  area 
varies  with  different  plants  and  is  char- 
acteristic of  any  given  species.  For  ex- 
ample, corn  is  typically  high  in  numbers 


[240] 


of  organisms  in  the  root  area,  while 
some  of  the  grasses  are  comparatively 
low.  Organisms  in  the  rhizosphere  are 
greatly  influenced  by  plant  roots  in  re- 
spect to  both  kinds  and  number,  and 
constitute  a  definite  and  unique  micro- 
population. 

Nutrient  Requirements 

Dependence  on  green  plants 

The  nutritional  requirements  of  mi- 
croorganisms are  much  the  same  as 
those  for  higher  plants.  Most  soil  micro- 
organisms except  algae,  however,  lack 
the  ability  exhibited  by  green  plants  to 
manufacture  organic  substances  from 
inorganic  elements  in  sunlight.  There- 
fore, the  majority  of  soil  microbes  are 
dependent  for  their  nutritional  and 
energy  sources  upon  the  decomposition 
of  the  complex  soil  organic  substances 
which  come  directly  or  indirectly  from 
green  plants.  In  attacking  organic  matter 
as  a  food  source,  soil  microorganisms 
bring  about  various  changes  which 
liberate  plant  food  in  forms  available  to 
higher  plants. 

Since  the  activities  of  soil  microor- 
ganisms  are  so  closely   associated  with 


but  completely  unavailable  to  others. 
The  soil  population  shifts,  a  succession 
of  different  forms  predominating  as  the 
food  materials  are  decomposed  and 
their  chemical  constitution  changed. 
Thus,  organic-matter  breakdown  sup- 
ports a  sequence  of  organisms,  each  ef- 
ficient in  utilizing  specific  degradation 
products.  Because  of  this  great  diversity 
of  organisms  a  soil  population  can  digest 
nearly  any  material  supplied  to  it.  When 
microorganisms  decompose  a  substrate, 
many  of  the  essential  elements  are  used 
to  form  the  material  of  their  developing 
cells.  These  materials  are  thus  rendered 
unavailable  to  the  plant  until  the  organ- 
isms die  and  are  decomposed  by  other 
microorganisms. 

Rate  of  decomposition 

As  an  example  of  the  rapidity  of  trans- 
formations by  organisms,  in  a  few  hours 
sugar  can  be  completely  decomposed  to 
carbon  dioxide  and  water,  or  protein  to 
ammonium,  carbon  dioxide,  water,  and 
other  compounds.  The  extent  to  which 
microorganisms  develop  is  limited  by 
the  nutrient  substrate,  environmental 
conditions,  and  the  formation  of  certain 
products    injurious    to    their    activities. 


the  growth  of  higher  plants,  efficient  soil      Although  modifications  in  the  supply  of 


treatment  to  kill  pathogens  requires  an 
understanding  of  these  activities  so  as 
to  capitalize  on  them.  Many  conditions 
resulting  in  unfavorable  plant  growth 
could  be  avoided  or  quickly  overcome 
if,  in  the  treatment  of  soils,  the  activities 
of  soil  microorganisms  were  considered 
to  the  extent  that  the  effects  on  higher 
plants  are. 


Steps  in  decomposition 
of  organic  matter 

The  complete  breakdown  of  organic 
matter  by  microbes  requires  numerous 
steps  and  many  types  of  organisms.  Soil 
organisms  differ  greatly  in  their  ability 
to  attack  a  given  substrate.  When  an 
organic  material  is  added  to  the  soil  it  is 
immediately  available  for  some  species, 


oxygen,  moisture,  and  inorganic  com- 
pounds, or  a  change  in  temperature  may 
affect  the  development  of  the  soil  popula- 
tion, the  greatest  response  in  activity  is 
accomplished  by  the  addition  of  organic 
substances  to  the  soil  or  bv  treatments 
which  destroy  part  of  the  soil  popula- 
tion. An  example  may  be  cited  from 
Waksman  and  Starkey  (1931),  who 
stated  that  when  a  ton  (dry  weight)  of 
fresh  organic  matter,  such  as  manures  or 
plant  stubble,  is  worked  into  soil  the 
microorganisms  immediately  become 
active.  First,  the  water-soluble  substances 
are  decomposed,  then  the  starches,  the 
proteins,  and  cellulose.  Within  10  to  20 
days,  under  favorable  conditions,  onlv 
about  1,000  to  1,200  pounds  may  be  left 
out  of  the  2,000  pounds  originally  added. 

[241] 


Our  experience  with  hoof  and  horn  meal, 
peat,  or  wood  shavings  indicates  a  much 
slower  breakdown  with  these  materials. 
This  is  one  of  the  reasons  why  leaf  mold 
and  manure  are  not  used  in  the  U.  C- 
type  mix. 

Carbon  nitrogen  ratio 

Much  can  be  interpreted  regarding 
end  products  which  will  be  formed  and 
the  speed  of  their  formation  if  the  organ- 
isms effecting  the  decomposition  of  an 
organic  material,  the  composition  of  the 
material,  and  the  environment  are 
known.  Fungi  use  the  organic  nutrients 
as  a  source  of  energy  and  of  carbon. 
They  use  for  synthesis  of  their  cells  an 
average  of  40  per  cent  (a  range  of  from 
10  to  65  per  cent  reported  by  most 
workers)  of  the  carbon  contained  in  the 
organic  compound  decomposed.  The  re- 
mainder of  the  carbon  goes  off  as  carbon 
dioxide  or  is  left  as  incompletely  de- 
composed material.  Bacteria  and  actino- 
mycetes  (in  the  presence  of  normal 
amounts  of  oxygen)  assimilate  an  aver- 
age of  25  per  cent  of  the  carbon  con- 
tained in  the  decomposed  organic  mate- 
rial (table  18). 

Accompanying  the  carbon  assimila- 
tion, appreciable  amounts  of  nitrogen 
are  used  in  the  formation  of  cell  pro- 
teins. The  carbon  content  of  microbial 
cells  averages  about  50  per  cent  of  the 
dry  weight.   The  nitrogen  contents  and 


the  carbon-to-nitrogen  ratios  of  the  vari- 
ous organisms  are  given  in  table  18. 

Effect  of  adding 
low-nitrogen  materials 

From  this  background  some  calcula- 
tions follow  which  give  the  microbio- 
logical basis  for  well-known  reactions 
observed  when  certain  materials  are 
added  to  soil. 

If  100  pounds  of  wood  shavings  con- 
taining 35  per  cent  carbon  and  0.2  per 
cent  nitrogen  are  added  to  soil,  the  fol- 
lowing calculations  indicate  the  results 
of  its  complete  decomposition  by  fungi: 

100  pounds  of  shavings  =  35  pounds 
of  carbon  and  0.2  pound  of  nitrogen 

35  pounds  of  carbon,  40  per  cent  as- 
similated by  fungi  =  14  pounds  of 
carbon  assimilated 

Ratio  of  carbon  to  nitrogen  in  fungi 
=10  to  1  ==  1.4  pounds  of  nitrogen 
assimilated. 

The  amount  of  nitrogen  furnished  by 
the  shavings  was  0.2  pound  and  that 
used  by  the  fungi  1.4  pounds;  therefore 
1.2  pounds  of  nitrogen  would  have  to  be 
supplied  to  the  fungi  for  the  decomposi- 
tion. Thus  the  soil  microorganisms 
would  be  competing  with  the  plant  for 
the  available  nitrogen  in  the  soil.  In  most 
cases  the  microorganisms  are  more  suc- 
cessful than  crop  plants  in  obtaining  the 
nitrogen  needed. 


Table  1 8.  Average  Nitrogen  and  Carbon  Contents  of  Organisms 


Fungi 

Bacteria 

Actinomycetes 

Mature  higher  plants 


Per  cent  of 

carbon  in 

nutrients  used 


40 
25 
25 


Nitrogen  con- 
tent of  the  cells, 
in  per  cent  of 
dry  wt. 


5 
10 
8.5 
1.5 


Ratio  of  carbon 

to  nitrogen  in 

the  cells 


10  to  1 

5  to  1 

6  to  1 
20  to  1 


*  Based  on  data  by  Waksman  and  Starkey  (1931). 


[242  1 


This  process  explains  the  temporary 
nitrogen  deficiency  noted  in  plants  after 
the  application  of  wood  shavings  and 
other  materials  low  in  nitrogen.  Experi- 
mental data  of  0.  A.  Matkin  (unpub- 
lished) show  that  1  bushel  of  sawdust 
can  completely  tie  up  the  available  nitro- 
gen for  6  weeks  when  %  pound  of  hoof 
and  horn  meal  is  added,  and  remove  the 
available  nitrates  for  the  same  period 
even  with  the  addition  of  3  pounds  of 
hoof  and  horn. 

Assimilation  of  nitrogen  by  microor- 
ganisms does  not  mean  that  nitrogen  is 
permanently  tied  up  in  microbial  cells. 
Microorganisms  are  relatively  short- 
lived (for  example,  bacterial  cells  may 
live  only  2  days,  those  of  fungi  several 
weeks),  and  upon  their  death  and  de- 
composition by  other  organisms  the 
nitrogen  and  other  elements  which  were 
assimilated  are  partially  released. 

The  temporary  nitrogen  deficiency 
caused  by  additions  of  low-nitrogen 
materials  can  be  overcome  by  the  addi- 
tion of  inorganic  nitrogen  fertilizers  or 
by  composting  the  organic  materials 
with  nitrogen  before  planting.  Compost- 
ing enables  the  material  to  be  decom- 
posed and  to  reach  a  relatively  stable 
state  before  its  use.  Composted  material 
does  not  support  the  rapid  growth  of 
organisms  resulting  in  nitrogen  competi- 
tion. There  are,  however,  numerous 
disadvantages  in  its  use  (Sec.  6). 

A  very  striking  example  of  rapid  mi- 
crobial use  of  nitrogen  occurs  with  the 
addition  of  sugar  to  soil.  Sugar  has  a 
high  carbon  percentage,  no  nitrogen, 
and  is  water-soluble.  A  small  amount 
added  to  soil  high  in  available  nitrogen 
will  so  enhance  microbial  development 
as  to  completely  tie  up  the  nitrogen  in 
less  than  2  days.  This  phenomenon  has 
been  used  in  the  laboratory  to  reduce 
the  soluble  nitrogen  in  pots  of  experi- 
mental plants,  with  rapid  and  complete 
results.  Some  growers  have  used  it  to 
prevent  soft  growth   of  their   plants  in 


recently  fertilized  soil  when  confronted 
with  an  unexpected  cloudy  period.  Most 
reports,  including  those  from  Matkin, 
agree  that  with  organic  materials  con- 
taining less  than  1.5  to  2  per  cent  nitro- 
gen a  deficiency  usually  results  upon 
decomposition. 

Effect  of  adding  materials  with 
over  2  per  cent  nitrogen 

Materials  containing  over  2  per  cent 
nitrogen  supply  more  than  is  required 
by  the  microorganisms.  This  excess  ap- 
pears as  ammonium  (later  oxidized  by 
bacteria  to  nitrate  under  many  condi- 
tions) . 

An  example  of  a  material  with  more 
than  2  per  cent  nitrogen  is  dried  blood 
meal.  If  completely  decomposed  by 
fungi,  the  following  calculations  would 
apply : 

100  pounds  of  dried  blood 

40  per  cent  carbon  =  40  pounds  of 

carbon 
13  per  cent  nitrogen  =  13  pounds  of 
nitrogen 
40  pounds  of  carbon,  40  per  cent  as- 
similated by  fungi  =  16  pounds  of 
carbon  assimilated. 
Ratio  of  carbon  to  nitrogen  for  fungi 
=  10  to  1  =  1.6  pounds  of  nitrogen 
assimilated. 
Nitrogen  available  for  crop,  11.4 
pounds. 
The   excess   nitrogen   would    appear    as 
ammonium    as   the   decomposition   pro- 
gressed. The  ammonium,  a  waste  product 
in  the  nutrition  of  the  fungi,  then  would 
be    available    to    higher    plants.    These 
processes   clarify   the   plant   stimulation 
that   occurs   when   organic   matter   with 
more  than  2  per  cent  nitrogen  is  added 
to  soil. 

Similar  calculations  could  be  made  to 
illustrate  the  results  of  the  same  decom- 
positions by  bacteria  or  actinomycetes. 
Bacteria  and  actinomycetes  assimilate 
less  carbon  than  fungi,  but  have  higher 
nitrogen  requirements. 


[243] 


The  cells  of  microorganisms  contain 
more  than  2  per  cent  nitrogen  and  thus 
when  they  die  and  are  decomposed  by 
other  organisms  nitrogen  becomes  avail- 
able. 

Effect  of  Environmental  Factors 

Temperature 

Since  the  availability  of  elements  from 
organic  materials  depends  upon  their 
liberation  by  the  activities  of  organisms, 
any  factor  affecting  the  growth  of  soil 
organisms  will  also  affect  the  availability 
of  such  plant  nutrients.  The  effect  of 
temperature  exemplifies  this  principle. 
At  optimum  temperatures  for  soil  organ- 
isms, organic  matter  is  broken  down 
most  efficiently.  The  process,  mentioned 
earlier,  of  using  sugar  on  soil  to  reduce 
available  nitrogen  proceeds  most  rapidly 
at  the  optimum  temperatures  for  the 
organisms  involved.  Many  growers  have 
noted  how  much  longer  it  takes  organic 
nitrogen  to  become  available  in  winter 
than  in  summer.  Soil  may  also  be  stored 
longer  at  lower  temperatures  before  the 
decomposition  of  organic  materials  re- 
leases a  toxic  concentration  of  nitrogen 
(Sec.  7). 

Oxygen  supply 

The  formation  of  peat,  an  important 
constituent  of  the  U.  C.-type  mixes,  illus- 
trates  the   effect   of   a   lack   of   gaseous 


oxygen  on  decomposition  by  organisms. 
The  saturation  with  water  of  the  mate- 
rial in  peat  bogs,  and  the  resulting  ex- 
clusion of  air,  prevents  the  complete 
oxidation  of  the  organic  matter.  Aerobic 
fungi  and  bacteria  cannot  function 
under  these  conditions,  and  the  decom- 
position of  organic  matter  is  dependent 
largely  upon  the  activities  of  anaerobic 
bacteria.  These  organisms  can  function 
without  oxygen  from  the  air,  but  can 
decompose  only  slight  amounts  of  such 
organic  constituents  as  waxes  and  lignin, 
while  other  constituents  are  broken 
down  very  slowly.  Peat  is  formed  as  the 
result  of  the  accumulation  of  resistant 
plant  residues  over  a  long  period  of  time 
in  an  air-free  habitat.  Once  formed,  it 
resists  decomposition,  even  when  ex- 
posed to  air.  Hence,  although  it  contains 
only  about  1  per  cent  nitrogen,  it  does 
not  cause  a  nitrogen  deficiency.  This  is 
one  reason  why  it  is  a  very  important 
constituent  in  the  U.  C.-type  mixes.  Red- 
wood sawdust  and  shavings  are  better 
for  use  in  U.  C.-type  mixes  than  are 
those  from  pine  and  fir,  for  the  same 
reason. 

These  examples  illustrate  the  impor- 
tance of  soil  organisms  and  the  processes 
they  perform.  By  understanding  that 
any  factor  affecting  growth  of  micro- 
organisms affects  plant  growth,  much 
mystery  in  growing  plants  is  explained. 


TRANSFORMATION  OF  PLANT  NUTRIENTS 
BY  MICROORGANISMS 


Nitrogen 

The  primary  source  of  soil  nitrogen  is 
the  atmosphere.  The  nitrogen  in  the  air, 
however,  is  in  the  form  of  an  inert  gas 
which  becomes  available  to  green  plants 
only  as  it  is  changed  into  combined 
forms  by  specific  soil  microorganisms  or 
chemical  processes.  Natural  electrical 
discharges  add  a  negligible  amount  of 


inorganic  nitrogen  compounds  to  soil. 
The  bulk  of  chemically  combined  nitro- 
gen is  produced  artificially  by  commer- 
cial fixation  processes,  generally  produc- 
ing ammonium  or  cyanamide.  The  main 
natural  means  by  which  plants  obtain 
nitrogen  from  the  atmosphere  is  as  a 
result  of  the  activities  of  certain  soil 
microorganisms. 


[244] 


Denitrification 

-^   Microorganisms 

NITRATE   NITROGEN]        Higher  p|anfs 

NO, 
available  to  plants 


Nonsymbiotic  and  symbiotic 
nitrogen-fixing  bacteria 


A 


"t 


Denitrification 
Bacteria    and    fungi 


Feed  on 


ORGANIC   NITROGEN 
in  protein  of  cells  of 
plants  and  microorganisms;   j         plant  material 
not  available  to  plants 


ORGANIC    NITROGEN 
_^       in  protein  of  cells 


of  animals; 


not  available  to  plants 

/ 


Specific  bacteria, 
Nitrobacter  spp. 


Various  organisms:  fungi, 
bacteria,  actinomycetes 


--> 


NITRITE   NITROGEN 
NO  2 
toxic  to  plants 


Nitrosomonas  spp. 


Specific  bacteria, 
Bacteria > 


11/ 


AMMONIUM   NITROi 
NH4 
available  to  plants 


Fig.  1 17.  Schematic  representation  of  the  most  important  nitrogen  transformations  in  soils. 


Nitrogen-fixing  bacteria 

As  seen  in  figure  117,  atmospheric 
nitrogen  is  fixed  into  microbial  cell 
material  by  free-living  bacteria.  This  re- 
action does  not  produce  appreciable 
amounts  of  nitrogen,  especially  in  soils 
with  growing  plants. 

Symbiotic  nitrogen-fixing  bacteria, 
which  live  in  the  root  nodules  of  legumes, 
cause  the  fixation  of  atmospheric  nitro- 
gen, which  becomes  available  to  their 
host  plant.  Symbiotically  fixed  nitrogen 
is  a  very  important  source  of  nitrogen, 
but  limited  to  the  Leguminosae. 

Ammonifiers  and  nitrifiers 

Whatever  the  source,  if  the  nitrogen 
is  tied  up  in  organic  compounds  (plant 
materials  or  animal  materials)  the  ac- 
tion of  soil  organisms  is  required  to 
convert  it  to  ammonium  or  nitrate  nitro- 


gen before  the  plant  can  utilize  it.  Some 
organisms  first  carry  the  reactions  from 
ammonium  to  nitrite,  and  then  others 
convert  from  nitrite  to  nitrate  nitrogen. 
Denitrification  (reduction  of  nitrate  to 
nitrite  and  then  to  gaseous  nitrogen) 
can  be  performed  by  many  organisms, 
but  is  not  important  where  soil  is  well 
aerated. 

The  essentials  of  nitrogen  conversion 
in  soils  are  summarized  as  shown  below. 
Organic  nitrogen  (unavailable  to  plants 
except  in  the  rare  uptake  of  amino 
acids)  is  broken  down  by  various  kinds 
of  organisms  to  produce  ammonium. 
Many  common  air-borne  fungi,  actino- 
mycetes, and  bacteria  (including  spore- 
formers)  are  capable  of  causing  this 
conversion. 

The  steps  from  ammonium  to  nitrate, 
on  the  contrary,  are  performed  by 
specific,      non-spore-forming      bacteria. 


Organic  nitrogen 

(unavailable) 


Type  1 
organisms 


Bacteria 

Fungi 

Actinomycetes 


Ammonium 

(available) 


Type  2 
organisms 


Specific 
bacteria 


Nitrate 

(available) 


[245] 


These  bacteria  are  among  the  most  sen- 
sitive of  soil  organisms.  Conditions  and 
treatments  (heat,  chemicals,  pH,  and  so 
on)  readily  withstood  by  the  ammonify- 
ing organisms  cause  injury  or  death  to 
the  nitrifying  bacteria. 

The  specificity  and  sensitivity  of  the 
nitrifiers,  as  opposed  to  the  abundant 
types  and  hardiness  of  the  ammonifying 
organisms,  account  for  the  fact  that  am- 
monium is  produced  under  a  much 
broader  range  of  conditions  than  is 
nitrate.  Conditions  and  treatments  which 
inhibit  the  nitrifiers  often  have  little 
effect  on  ammonium  production.  For 
example,  highly  acid  media  inhibit  nitri- 
fication much  more  than  ammonium 
formation,  as  illustrated  in  the  following 
data  from  Section  7. 

In  a  growing  medium  at  pH  3.9  the 
total  ammonium  and  nitrate  nitrogen 
was  251  ppm,  of  which  only  26  ppm  was 
in  the  nitrate  form.  However,  at  pH  5.9, 
with  a  total  of  505  ppm,  350  ppm  ap- 
peared in  the  nitrate  form.  Thus  the 
medium  at  pH  5.9  had  about  twice  as 
much  total  available  nitrogen  but  nearly 
fourteen  times  as  much  in  the  nitrate 
form  as  in  the  medium  at  pH  3.9. 

Low  temperatures  also  reduce  nitrifi- 
cation much  more  than  ammonification. 
Under  most  environments  some  organ- 
isms included  in  the  ammonifying  popu- 
lation can  grow  at  temperatures  that 
inhibit  the  specific  bacteria  concerned 
with  nitrification.  In  normal,  untreated 
field  soil,  however,  the  bulk  of  nitrogen 
available  to  the  plant  appears  in  the 
nitrate  form.  Ammonium  formation 
usually  becomes  a  factor  only  after  some 
type  of  soil  treatment,  as  is  considered 
later  in  this  section. 

Carbon 

Carbon  makes  up  an  average  of  ap- 
proximately 50  per  cent  of  the  dry 
weight  of  all  chemical  elements  in  plant 
and  animal  tissues.  Carbon  dioxide  gas 
ia  the  source  of  carbon  for  the  growth  of 
green    plants.   Animals  derive  their  car- 


bon from  plant  materials.  Thus,  the 
primary  source  of  carbon  for  plant  and 
animal  life  is  carbon  dioxide  gas. 

Carbon  dioxide  is  present  in  the  at- 
mosphere in  a  concentration  of  0.03  per 
cent,  and  about  %5  of  the  total  carbon 
content  of  the  atmosphere  is  consumed 
each  year  by  the  plant  world.  This  sup- 
ply is  never  exhausted,  however,  largely 
because  it  is  constantly  being  replen- 
ished by  the  microbial  decomposition  of 
organic  substances  in  the  soil,  but  also 
by  means  of  plant  and  animal  respira- 
tion and  industrial  burning.  When  mi- 
croorganisms completely  decompose  an 
organic  material,  the  carbon  goes  off  as 
carbon  dioxide. 

Because  of  the  activities  of  soil  mi- 
crobes, the  atmosphere  of  the  soil  con- 
tains from  20  to  200  times  as  much 
carbon  dioxide  as  air.  This  high  carbon 
dioxide  content  of  the  soil  results  in  the 
formation  of  carbonic  acid,  which  aids 
in  bringing  insoluble  elements  such  as 
phosphorus  into  solution.  The  produc- 
tion of  carbon  dioxide  from  soil  is,  in 
fact,  often  used  as  a  measure  of  the 
activity  of  the  soil  microbial  population. 
The  transformations  of  carbon  are  sum- 
marized in  figure  118. 

Sulfur 

Sulfur  is  another  element  essential  to 
plant  growth  that  undergoes  microbial 
transformation.  Sulfur  reaches  soil  as 
organic  compounds  (plant  and  animal 
residues),  elementary  sulfur  (fertilizers, 
fungicides,  soil  amendments),  or  sul- 
fates (fertilizers,  amendments,  irriga- 
tion water) . 

The  transformation  of  sulfur-contain- 
ing organic  compounds  resembles  that 
of  nitrogen.  Instead  of  ammonium 
(NH4),  hydrogen  sulfide  (H2S)  is 
formed,  and  through  various  reactions 
sulfate  (SO.,)  is  produced.  Bacteria 
known  as  "sulfur  bacteria"  are  respon- 
sible for  the  rapid  conversion  of  ele- 
mental sulfur  to  sulfuric  acid. 


[  246  ] 


/* 


Atmospheric 
Carbon  Dioxide 

co2 


Photosynthesis 


t 


Respiration 


Microbial 
Decomposition 


Microbial 

Decomposition 


Respiration 


Fig.  1 18.  The  carbon  cycle. 


Plants 

Organic 

Compounds 


Animals  feeding 
on  plants 


I 


Animals 

Organic 

Compounds 


j 


This  change  (oxidation)  not  only 
renders  the  sulfur  available  for  plants 
but  makes  the  soil  reaction  more  acid. 
The  reaction  is  made  use  of  in  reclaim- 
ing alkali  soils,  to  reduce  certain  plant 
diseases  such  as  potato  scab,  to  increase 
iron  and  phosphate  availability,  and  to 
make  soil  slightly  acid  or  neutral  for  the 
growth  of  certain  plants.  One  pound  of 
soil  sulfur  when  oxidized  to  sulfate  by 
organisms  produces  about  3  pounds  of 
sulfuric  acid. 

Phosphorus 

Phosphorus,  essential  for  plant  growth, 
is  found  in  soil  organic  compounds  or 
as  insoluble  phosphates.  Several  bacteria 
and  fungi  can  liberate  phosphorus  from 
organic  compounds  in  an  inorganic 
form.  Phosphorus  is  a  constituent  of  mi- 
crobial cells  and,  just  as  with  nitrogen, 
may  be  rendered  temporarily  unavail- 
able to  plants  when  materials  low  in 
phosphorus  are  rapidly  decomposed. 

Insoluble  phosphates  are  made  avail- 
able mainly  by  the  indirect  action  of 
microorganisms.  Many  of  the  organic 
and   inorganic    acids   produced   by   soil 


microbes  react  with  the  insoluble  phos- 
phates to  form  soluble  compounds.  Ger- 
retsen  (1948)  has  shown  that  organisms 
in  the  rhizosphere  have  considerable 
solvent  action  on  insoluble  phosphates. 
In  the  past  this  has  been  attributed  to  the 
roots  themselves. 

Other  Essential  Elements 

Other  essential  elements  such  as  potas- 
sium, calcium,  magnesium,  and  iron  are 
affected  either  directly  or  indirectly  by 
soil  microorganisms.  When  organic  sub- 
stances are  decomposed  by  microbes 
they  release  potassium,  which  is  then 
available  to  higher  plants.  Although 
potassium  is  usually  added  to  the  soil 
in  a  soluble  form,  organic  acids  from 
microorganisms  help  liberate  it  when  it 
becomes  fixed  by  the  soil.  Calcium,  mag- 
nesium, and  iron  are  affected  indirectly 
by  the  actions  of  soil  organisms,  espe- 
cially through  acid  production. 

Many  experimenters  have  shown  that 
soil  organisms  can  affect  manganese 
nutrition  of  plants.  Manganese-deficiencv 
symptoms  can  result  from  the  action  of 
certain      microorganisms     that     render 


[247] 


manganese  insoluble  by  oxidizing  it. 
Treatments  which  eliminate  the  man- 
ganese-oxidizing organisms  alleviate  the 
deficiency. 

A  similar  situation  was  reported  in 
California  in  the  little-leaf  rosette  disease 
of  peaches,  attributable  to  zinc  defi- 
ciency. Ark  (1937)  found  high  bacterial 
concentrations  in  the  root  zone  of  sus- 
ceptible trees,  but  overcame  the  trouble 


by  applying  zinc  salts  or  by  sterilizing 
the  soil. 

Most  elements  essential  to  plant 
growth  are  also  required  by  soil  organ- 
isms. This  direct  competition,  along  with 
indirect  effects,  such  as  release  of  acids, 
so  closely  links  the  activities  of  these  two 
groups  that  any  change  in  the  microbial 
population  has  an  effect  on  plant  nutri- 
tion. 


EFFECT  OF  SOIL  TREATMENTS  ON  MICROORGANISMS 


One  of  the  important  considerations 
in  the  increasing  use  of  soil  treatments 
by  chemicals  and  steam  is  their  effect  on 
the  population  of  soil  organisms,  both 
harmful  and  beneficial.  When  soil  is 
treated  with  the  recommended  chemicals 
(Sec.  11)  or  steam  (sees.  8  and  9)  many 
excellent  results  are  achieved.  Soil-borne 
plant  pathogens  are  controlled,  weeds 
and  insects  are  eliminated,  and  very 
marked  plant-growth  increases  result 
(fig.  119).  Treatment  of  soils  is  neces- 
sary because  of  these  many  advantages, 
but  the  effect  on  beneficial  soil  organ- 
isms may  be  disadvantageous. 


decontamination  Hazard 

When  soil  is  treated,  the  number  of 
soil  microorganisms  is  greatly  reduced 
for  the  first  few  days;  then  it  rises  and 
eventually  exceeds  that  of  untreated  soil. 
Let  us  consider  what  occurs  when  the 
soil  is  treated,  and  disease  organisms 
gain  access  to  this  soil.  The  treatment 
destroys  a  large  part  of  the  dense  popu- 
lation of  soil  microbes,  and  the  first 
organisms  to  return  after  treatment  meet 
no  severe  competition.  Thus,  if  plant 
pathogens  are  among  the  first  to  re- 
colonize  the  soil,  they  develop  rapidly, 


.■:■■;:■  ,,; 


Fig.  119.  Increased  growth  of  tobacco  plants  in  chloropicrin-treated  soil  as  compared  to  un- 
treated soil  (left). 


[  248  ] 


Fig.  120.  The  effect  of  steaming  field  soil  on  plant  growth  and  disease  spread.  A,  Steamed 
soil;  excellent  stand  and  growth.  B,  Nontreated  soil;  poorer  stand  and  growth  than  in  A. 
C,  Steamed  soil  with  Rhizoctonia  added  at  arrows,  showing  greater  pre-  and  postemergence 
damping-off  than  in  nonsteamed  soil.  D,  Nonsteamed  soil  with  Rhizoctonia  added  at  arrows. 


and  cause  severe  disease  losses.  It  there- 
fore is  important  to  the  grower  that 
pathogens  do  not  gain  entrance  to 
treated  soil.  Benches,  flats,  equipment, 
seed,  and  the  numerous  other  sources  of 
pathogen  introduction   (Sec.  3)   become 


Danger  of  Inadequate  Treatment 

One  source  of  trouble  is  the  use  of 
treatments  which  destroy  a  portion  of 
the  soil  population,  but  leave  pathogens 
unharmed.  Severe  losses  often  occur 
after  such  treatments,  because  of  the  in- 


potentially  more  dangerous  when  using      crease  of  the  surviving  pathogens  under 


treated  as  compared  with  untreated  soil. 
The  damping-off  of  seedlings  in  nur- 
sery soils  in  California  caused  by  the 
soil  fungus  Rhizoctonia  solani  may  be 
cited  as  an  example  (Sec.  3).  This  path- 
ogen can  be  effectively  eliminated  from 
soil  by  any  of  several  treatments.  If, 
however,  the  fungus  then  gets  into  this 
treated  soil,  the  resulting  loss  is  much 
more  severe  than  that  suffered  in  un- 
treated soil  (fig.  120). 


the  conditions  of  decreased  competition. 
The  two  most  common  ways  of  creating 
the  above  condition  are: 

1.  The  use  of  fungicidal  treatments  at 
lower  than  recommended  rates. 

2.  The  use  of  treatments  that  control 
other  pests  or  specific  diseases  and 
may  markedly  decrease  the  soil  popu- 
lation, but  not  destroy  certain  plant 
pathogens. 

In  both  cases  the  loss  results  from  the  dis- 
ruption of  the  balance  in  the  soil  popula- 


[249] 


tion  and  the  shift  to  conditions  favoring 
the  increase  of  a  plant  pathogen. 

Examples  of  damage  due  to  treatments 
at  too  low  a  dosage  are  most  often  seen 
with  treatment-resistant  pathogens  such 
as  the  soil  fungus  Verticillium  albo- 
atrum.  V erticillium  is  relatively  hard  to 
kill  and  therefore  it  is  possible  to  apply 
treatments  which  remove  much  of  the 
soil  population,  leaving  V erticillium  to 
infect  more  severely.  Where  V erticillium, 
Fusarium,  or  other  more  resistant  fungi 
are  a  problem  it  is  dangerous  to  use  less 
than  recommended  dosages  for  their 
control.  Also,  if  treatments  for  control 
of  these  diseases  are  done  improperly  or 
incompletely,  the  same  severe  loss  can 
occur  as  from  too  low  a  dosage. 

Examples  of  the  second  case  are  also 
encountered  where  treatments  for  given 
pests  enhance  the  damage  due  to  other 
pathogens.  Treatments  used  for  the  con- 
trol of  nematodes  have  sometimes  re- 
sulted in  increased  losses  from  Verticil- 
Hum  wilt.  One  experimental  fungicide 
recommended  specifically  for  Rhizoc- 
tonia  control  has  been  reported  to  con- 
trol Rhizoctonia,  but  heavy  losses  due  to 
water  molds,  not  important  before  treat- 
ment, may  then  be  sustained. 

These  examples  are  cited  to  illustrate 
the  role  soil  microorganisms  play  in 
disease  control,  and  the  importance  of 
considering  them  in  all  control  opera- 
tions. 

Increased  Need  for  Sanitation 

The  proved  advantages  and  wide- 
spread  use   of  soil  treatments  with  the 


resulting  increased  hazard  and  severe 
effects  of  recontamination,  point  to  the 
need  of  a  more  thorough  knowledge  of 
the  sources  and  means  of  combating 
pathogen  introduction.  The  most  impor- 
tant means  of  eliminating  sources  of  con- 
tamination is  a  vigorous  and  constant 
program  of  sanitation.  To  reap  the  full 
benefits  of  soil  treatment  the  grower 
must  ever  be  on  guard  to  protect  the 
treated  soil.  Every  operation  must  be 
checked  for  the  possible  introduction  of 
disease  material  (see  "A  Nursery  Sanita- 
tion Code"  in  Sec.  1).  The  chain  of  suc- 
cessful growing,  though  containing 
strong  links  of  adequate  nutrition, 
disease-free  soil,  and  clean  stock,  can 
easily  break  if  a  weak  link,  such  as  the 
presence  of  pathogens  on  flats  or 
benches,  exists. 

The  alert  grower  can  more  effectively 
combat  recontamination  if  he  under- 
stands something  about  the  sources  of 
disease  organisms  and  the  practices 
which  introduce  them  into  soil.  The  two 
main  channels  of  introducing  pathogens 
into  pathogen-free  nursery  soil  are 
planting  stock  and  infested  soil  or  plant 
particles  which  may  come  into  contact 
with  the  clean  soil  in  many  ways.  The 
most  important  plant  pathogens  causing 
damping-off,  root  rots,  and  related 
diseases  are  not  air-borne,  but  must  de- 
pend upon  the  mechanical  transfer  of 
infested  soil  or  water  and  infected  plant 
tissue  for  their  spread  (Sec.  3) .  This  fact 
enables  the  alert  grower  to  reduce  or 
eliminate  the  transfer  of  infested  mate- 
rial by  careful  practices. 


CONTROLLED  COLONIZATION:  A  FUTURE  STEP 


To  Retard  Pathogens 

Since  soil  treatment  has  proved  so 
advantageous  and  is  so  widely  used,  and 
since  contamination  of  treated  soil  may 
be  severe,  there  is  need  for  a  method  of 
protecting  treated  soil  from  introduced 
disease  organisms.  In  an  effort  to  meet 


this  need,  studies  were  conducted  by  the 
author,  using  organisms  antagonistic  to 
plant  pathogens  but  without  adverse 
effect  on  plants.  The  purpose  of  report- 
ing this  work  is  not  to  make  any  recom- 
mendations at  this  time,  but  to  provide 
advance  information  on  some  steps  be- 


I  250  1 


ing  taken  toward   a   future  solution   of 
soil-disease  problems. 

The  retardant  organisms,  singly  or  in 
groups,  are  added  to  soil  immediately 
after  treatment.  Since  they  are  the  first 
to  return,  they  make  rapid  growth,  just 
as  would  pathogens  if  they  were  the  first 
organisms  returning  after  treatment. 
These  beneficial  or  pathogen-retarding 
organisms  colonize  the  soil  and  protect 
it  from  recontamination.  They  act  either 
as  antibiotic  producers  or  as  parasites  or 
competitors  of  pathogens  or  in  various 
combinations  of  these  types.  The  con- 
tinual production  of  antibiotics  in  the 
soil  by  organisms  appears  more  effective 
than  the  addition  of  antibiotics  alone, 
since  these  chemicals  break  down 
rapidly.  The  effect  of  several  different 
organisms  in  restricting  the  spread  or 
completely  stopping  damping-off  due  to 
Rhizoctonia  solani  is  illustrated  in  figures 
121  and  122.  A  retardant  organism  was 
added  to  flats  of  steamed  U.  C.-type  mix 
at  the  time  of  seeding.  The  flats  were 
also  inoculated  with  Rhizoctonia  in 
heavy  enough  concentration  to  cause 
eventual  100  per  cent  loss  in  flats  not 
protected  with  retardant  organisms. 

Complications 

The  results  are  very  encouraging,  but 
some  of  the  complications  should  be  dis- 
cussed. The  Rhizoctonia-r  etar  ding  effect 
appears  to  last  for  the  entire  susceptible 
seedling  stage  with  some  organisms,  but 
is  only  temporary  with  others.  The  re- 
tarding effect  of  Myrothecium  (a  com- 
mon soil  fungus)  diminished  after  a 
month  in  flats  with  growing  plants.  In- 
creased concentrations  of  Myrothecium 
high  enough  to  inhibit  Rhizoctonia  for 
longer  periods  stunted  the  pepper  seed- 
lings. Both  this  stunting  effect  (fig.  123) 
and  the  Rhizoctonia-retarding  effect 
could  be  enhanced  by  the  addition  of 
various  organic  amendments.  Since 
Myrothecium  is  a  rapid  cellulose  decom- 
poser, materials  high  in  cellulose  gave 
the  greatest  effect.  This  case  serves  to 


Fig.  121.  The  effect  of  adding  Rhizoctonia 
with  and  without  a  retardant  (Myrothecium)  to 
flats  of  a  steamed  U.  C.-type  mix  planted  to 
peppers.  A,  (upper  right)  Rhizoctonia  plus  My- 
rothecium added  at  this  point;  damping-off 
prevented.  B,  (lower  left)  Rhizoctonia  added 
at  this  point;  damping-off  severe  and  continu- 
ing. 

illustrate  the  sequence  of  organisms  in 
soil  resulting  from  the  addition  of  or- 
ganic matter.  When  cellulose-rich  mate- 
rial was  first  added,  the  Myrothecium 
population  increased  rapidly,  with  re- 
sulting retarding  effect  on  Rhizoctonia. 
As  the  cellulose  became  decomposed  the 
population  shifted,  and  with  other  or- 
ganisms becoming  dominant  the  retard- 
ing effect  of  Myrothecium  on  Rhizoc- 
tonia diminished. 

The  soil  pH  has  a  marked  influence  on 
the  organisms  used  in  controlled  coloni- 
zation. In  the  acid  range,  fungi  such  as 
species  of  Penicillium  and  Trichoderma 
are  most  effective,  while,  as  the  reaction 
approaches  neutrality,  species  of  the 
genus  Streptomyces  become  more  prom- 
ising. 

Competition  escape  accounts  for  one 
of  the  difficulties  encountered  in  the  con- 
trolled colonization  work.  A  soil  may  be 
colonized  with  a  Rhizoctonia-retarding 
organism  to  the  extent  that  growth  of 
Rhizoctonia  is  completely  restricted.  If 
this  same  soil  contains  growing  plants, 
Rhizoctonia  may  spread  from  plant  to 


[251] 


Fig.  122.  Protection  from  Rhizoctonia  damping-off  achieved  by  the  addition  of  a  retarding 
organism  to  flats  of  a  steamed  U.  C.-type  mix  planted  to  peppers.  A,  Control  flat;  no  retardant 
or  Rhizoctonia  added.  B,  Trichoderma  sp.  added  as  a  retardant  to  the  whole  flat,  and  Rhizoctonia 
added  at  arrow;  resulted  in  complete  protection  comparable  to  control  flat.  C,  Penicillium  sp. 
added  as  a  retardant  to  the  whole  flat,  and  Rhizoctonia  added  at  arrow;  resulted  in  a  small 
area  of  preemergence  damping-off,  but  no  postemergence  damping-off.  D,  Rhizoctonia  alone 
added  at  arrow;  resulted  in  complete  loss. 


plant  above  the  soil  surface.  Thus,  under 
conditions  of  plant  crowding  and  high 
humidity,  Rhizoctonia  may  escape  the 
retarding  effect  of  the  soil  flora. 

The  above  complications  do  not  mean 
that  it  will  not  prove  feasible  to  use  con- 
trolled colonization  to  protect  against 
disease  loss,  but  indicate  the  need  for 
further  study  and  knowledge  of  soil 
microorganisms  in  relation  to  plants. 

Prerequisites  for  the  program 

The  entire  concept  of  biological  con- 
trol of  soil-borne  plant  pathogens  shows 
increasing  promise,  and  the  nursery  in- 
dustry is  unique  in  having  available  all 
the  features  requisite  for  a  successful 
program  using  beneficial  organisms. 


Before  the  addition  of  organisms  to 
soil  can  be  effective  in  protecting  the  soil 
from  subsequent  contamination  by  path- 
ogens,, most  of  the  existing  soil  micro- 
flora must  be  destroyed  or  the  existing 
organism  balance  changed  in  some  other 
way.  This  is  already  accomplished  in 
most  nurseries  by  either  steam  or  chemi- 
cal soil  treatment. 

Consistent  protection  by  beneficial 
organisms  is  also  dependent  upon  a 
uniform  soil.  This  is  most  satisfactorily 
obtained  by  the  use  of  a  U.  C.-type  soil 
mix,  which  can  be  accurately  duplicated. 

Finally,  the  controlled  and  stable  con- 
ditions of  nursery  growing  further  add 
feasibility  to  obtaining  positive  results 
with  biological  control.  California  nur- 


[  252  ] 


sery  operations  so  closely  parallel  the 
laboratory  conditions  used  in  these 
studies  that  results  in  protecting  soil 
against  pathogens,  such  as  those  illus- 
trated above,  appear  to  be  entirely  pos- 
sible. No  major  modifications  of  the 
ultimate  techniques  developed  in  the  re- 
search laboratory  should  be  necessary 
for  commercial  application. 

Untreated  soil  contains  such  an  abun- 
dance of  life  that  little  is  gained  by 
adding  organisms  without  some  type  of 
treatment  to  overcome  the  natural 
biological  buffering  capacity  of  the  soil. 
This  can  be  accomplished  by  treatment 
to  alter  the  soil  population,  or  by  sup- 
plying a  specific  nutrient  substrate  to 
make  a  favorable  environment  for  the 
desired  organisms.  Other  environmental 
factors  such  as  pH,  temperature,  and 
moisture  can  also  influence  the  popula- 
tion and  can  be  controlled.  It  is  impor- 
tant   that    the    grower    realize    that    he 


cannot  simply  add  an  organism  to  the 
soil  and  expect  it  to  have  some  desirable 
effect. 

To  Improve  Nitrogen  Nutrition 

In  addition  to  the  effect  on  disease, 
removal  of  beneficial  soil  organisms  can 
adversely  affect  the  nitrogen  nutrition  of 
plants.  Two  points  are  considered: 

1.  An  initially  low  level  of  available 
nitrogen  after  treatment. 

2.  The  bulk  of  available  nitrogen  after 
treatment  appears  in  the  ammonium 
form.  In  general,  plants  continually  sup- 
plied with  ammonium  synthesize  organic 
nitrogen  rapidly  and  thus  tend  to  deplete 
their  supply  of  sugars  and  starch  (Sec. 
7).  An  undesirable  condition  of  carbo- 
hydrate deficiency  may  result  in  softer 
and  more  succulent  plants,  as  in  the  case 
of  snapdragon  seedlings.  Often  plants 
in  this  condition  cannot  survive  trans- 
planting. Nitrate  is  absorbed  and  used 


Fig.  123.  The  effect  of  amendments  in  increasing  both  the  retarding  effect  on  Rhizoctonia  and 
the  stunting  of  plants  when  added  with  Myrothecium  to  flats  of  a  steamed  U.  C.-type  mix  planted 
to  peppers.  Left,  sterilized  wheat  straw  added  to  upper  half,  no  amendment  to  lower  half  of 
flat.  Note  greater  restriction  of  Rhizoctonia  damping-off  with  the  wheat  straw  amendment.  Right, 
sterilized  pepper  seeds  added  to  upper  half,  no  amendment  to  lower  half  of  flat.  The  three 
representative  seedlings  on  each  card  show  the  greater  stunting  of  pepper  seedlings  when  sterile 
pepper  seeds  are  added. 


[253] 


comparatively  more  slowly,  and  plants 
supplied  this  form  of  nitrogen  usually 
form  more  lignin  and  cellulose,  the 
major  constituents  of  the  mechanical 
tissues  of  plants.  A  few  plants  apparently 
require  at  least  part  of  their  nitrogen  in 
the  nitrate  form  for  best  growth.  Am- 
monium can  also  be  toxic  if  high  enough 
concentrations  are  reached  (Sec.  7). 

As  explained  earlier,  organisms  are 
needed  to  convert  unavailable  organic 
nitrogen  into  an  available  form.  The 
initial  reduction  in  all  organisms  im- 
mediately after  treatment  accounts  for 
the  initial  lag  in  available  nitrogen  from 
organic  sources.  A  partial  alleviation  of 
this  low  initial  nitrogen  level  could  be 
achieved  by  the  introduction  of  organ- 
isms especially  efficient  in  converting  un- 
available organic  nitrogen  into  available 
forms.  Experiments  with  several  soil 
fungi  have  shown  this  approach  to  be 
feasible.  The  use  of  nitrate  starter  solu- 
tions is  also  helpful  (Sec.  7). 

Introducing  nitrifiers 

After  steaming,  the  predominant 
available  nitrogen  is  in  the  ammonium 
form,  owing  to  the  abundance  and  re- 
sistance of  type-1  organisms  as  compared 
with  those  of  type  2  (p.  245).  Further- 
more, the  type-1  organisms  include 
many  common  air-borne  fungi,  actino- 
mycetes,  and  bacteria.  These  organisms 
colonize  treated  soil  with  comparative 
rapidity  and  some  of  the  spore-forming 
bacteria  survive  most  treatments.  In  con- 
trast, the  type-2  or  nitrifying  organisms 
are  readily  killed  by  most  treatments  and 
are  much  fewer  in  species  and  numbers. 
Thus,  the  nature  of  the  organisms  in- 
volved explains  the  ammonium  accumu- 


lation, and  also  indicates  a  method  of 
carrying  the  reaction  on  to  the  forma- 
tion of  nitrate  nitrogen  by  adding  the 
appropriate  organisms  to  soil.  This  has 
been  accomplished  in  the  University 
laboratories  in  Los  Angeles  by  adding 
to  the  steamed  mix  a  water  suspension  of 
soil  containing  nitrifying  organisms.  As 
discussed  in  Section  7,  in  21  days  the 
nitrate  nitrogen  in  one  series  with  in- 
oculated soil  averaged  101  ppm  com- 
pared to  8  ppm  in  soil  not  inoculated 
with  nitrifying  organisms.  To  be  com- 
pletely practical  for  all  cases  the  time 
required  to  reach  the  21-day  level  should 
be  reduced,  but  these  tests  show  the  po- 
tential value  of  controlling  soil  nitrogen 
by  adding  certain  organisms. 

A  Possible  Future  Program 

A  future  program  for  a  grower  of 
plants  in  containers  may  be  envisioned 
from  these  facts.  Flats,  pots,  and  other 
containers  filled  with  a  U.  C.-type  soil 
mix  are  treated  with  steam  or  chemicals 
to  remove  all  organisms,  and  then  a 
suspension  of  organisms  is  sprayed  on 
the  flats  before  planting.  This  suspension 
would  include  organisms  capable  of  sup- 
pressing accidentally  introduced  patho- 
gens. Also  included  would  be  organisms 
which  would  promote  the  early  produc- 
tion of  nitrate  nitrogen.  This  program 
would  greatly  reduce  or  eliminate  soil- 
borne  disease  problems  and  would  give 
plants  grown  in  this  way  a  measure  of 
protection  even  if  set  out  for  growing  in 
untreated  field  soil.  Plant  nutrition  when 
organic  nitrogen  is  used  would  be 
greatly  improved.  With  all  the  special 
features  of  nursery  growing  in  a  U.  C.- 
type  soil  mix,  this  program  is  a  definite 
possibility. 


[254] 


SECTION 


Importance  of  Variation 
and  Quantity  of  Pathogens 


Richard  D.  Durbin 


The  variability  of  plant  pathogens 
The  inoculum  potential 


Infected  seed  and  stock 


Longevity  in  soil 


Mixed  infections 


Obligation  of  the  nursery 


rowers  often  ask,  "Since  rhizoc  is 
already  present  in  my  soil,  what  dif- 
ference does  it  make  if  I  introduce  more 
of  it?"  It  is  commonly  assumed  that  be- 
cause a  fungus  is  present  in  the  soil,  the 
inadvertent  addition  of  more  of  it  with 
infected  nursery  stock,  seed,  bulbs,  or  in- 


fested soil  will  make  little  or  no  dif- 
ference in  crop  losses.  This  mistaken  rea- 
soning can  have  serious  consequences; 
it  may  lead  to  loss  of  the  given  crop  and 
infestation  of  the  field,  ruining  it  for  cer- 
tain crops. 


THE  VARIABILITY  OF  PLANT  PATHOGENS 


The  variability  of  living  organisms  is 
quite  generally  accepted.  Every  indi- 
vidual in  a  given  species  is  different  in 
many  respects  from  all  the  rest,  yet 
shares  certain  common  characters  with 
the  other  members  of  the  species.  Thus 
we  say  that  plants  and  animals  having 
the  same  scientific  or  common  name  are 
similar  but  not  identical.  All  humans 
are  classified  as  Homo  sapiens,  although 
we  have  only  to  look  around  to  see  that 
each  is  unique  in  some  respect.  In  flower- 


ing plants  man  has  taken  advantage  of 
some  of  the  apparent  differences  within 
crop  species  to  develop  varieties  which 
are  outstanding  in  yield,  quality,  or 
adaptability  to  certain  environments. 

Lower  plants,  such  as  fungi  and  bac- 
teria, also  exhibit  this  characteristic 
variation,  but  it  is  not  so  evident  because 
of  their  small  size.  Variation  in  these 
microorganisms  may  be  evident  in  the 
things  they  are  able  to  do,  that  is,  in 
their    physiological    activities;     for    in- 


[255] 


stance,  they  may  produce  more  or  a 
slightly  different  form  of  a  substance 
such  as  penicillin  or  streptomycin.  While 
this  fact  has  been  of  great  benefit  to  in- 
dustry, it  has  seriously  complicated  the 
prevention  of  diseases  in  plants  and 
animals. 

Since  the  ability  to  produce  plant  dis- 
ease involves  the  interaction  of  complex 
physiological  systems  of  pathogen  and 
host,  it  is  not  surprising  that  one  also 
finds  variation  here.  The  extensive  varia- 
tion among  crop  plants  is  not  considered 
in  this  section,  but  some  mention  of 
parasite  variation  is  pertinent.  This  sub- 
ject is  of  concern  to  both  grower  and 
pathologist. 

Host  Range  and  Virulence 

Fungi 

The  fungus  Rhizoctonia  solani  con- 
tains individuals  or  strains  which  may 
vary  in  host  range,  pathogenicity  on 
any  one  host,  and  response  to  the  en- 
vironment. Each  strain  is  able  to  attack 
a  given  group  of  plants,  with  the  patho- 
genicity of  many  strains  overlapping 
on  a  single  host.  One  strain  may  be 
strongly  virulent  to  pepper  but  unable 
to  attack  Tagetes,  whereas  another  may 
attack  Tagetes  vigorously;  some  strains 
from  tomato  are  virulent  to  bean  while 
others  are  not.  Many  such  examples 
exist.  On  any  particular  host  plant  the 
disease  produced  may  vary  from  stunt- 
ing to  complete  loss  from  damping-off, 
according  to  the  strain  involved. 

In  one  test  on  variability  in  patho- 
genicity, eleven  isolates  of  Rhizoctonia 
solani  were  compared  in  virulence  to 
pepper  seedlings;  figure  124  shows  some 
typical  results.  Although  pathogenic  to 
the  source  host  in  each  case,  many  of 
the  isolates  were  not  pathogenic  to 
pepper,  while  others  were  more  damag- 
ing than  was  one  originally  from  pepper. 
These  differences  in  virulence  among 
the  isolates  exemplify  the  danger  of  in- 
troducing additional  Rhizoctonia  into  an 


already  infested  soil.  Why  take  the 
chance? 

Some  strains  of  Rhizoctonia  solani  are 
almost  saprophytic  and  cause  little  plant 
damage,  such  as  those  commonly  pro- 
ducing black  sclerotia  ("the  dirt  that 
won't  wash  off")  on  potato  tubers.  Dif- 
ferent strains  are  now  known  to  cause  a 
serious  potato  stem  rot,  although  for 
many  years  they  were  considered  to  be 
the  same  as  the  tuber-attacking  forms. 
Tubers  were  even  treated  with  fungicides 
to  eradicate  the  sclerotia  in  the  hope  of 
stopping  the  stem  rot,  until  it  was  dis- 
covered that  the  diseases  were  different. 
Still  other  strains  of  R.  solani  are  re- 
stricted to  the  above-ground  environ- 
ment, and  cause  foliar  blights  of  various 
crops  in  the  southeast  United  States.  In 
southern  California  we  are  most  familiar 
with  strains  causing  root  rots  and  damp- 
ing-off of  nursery  crops. 

Stephen  Wilhelm1  has  found  that 
Verticillium  albo-atrum  includes  forms 
varying  from  those  apparently  unable  to 
invade  the  host,  through  those  which 
may  invade  without  producing  the  dis- 
ease, to  those  which  invade  and  produce 
varying  degrees  of  disease  on  a  given 
host.  Some  forms  that  do  little  harm  on 
one  host  may  be  severe  on  another.  Thus, 
among  isolates  pathogenic  to  stock  some 
are  nonpathogenic  on  tomato,  others  in- 
vade tomato  roots  but  cause  no  above- 
ground  disease  symptoms,  while  still 
other  isolates  are  severely  pathogenic  to 
tomato.  Furthermore,  Wilhelm  and 
Raabe  (1956)  have  recently  found  a 
strain  of  Verticillium  albo-atrum  that 
produced  wilt  on  the  Manetti  rose  root- 
stock,  formerly  resistant  to  known  strains 
of  this  fungus. 

The  genus  Fusarium  includes  types 
which  live  saprophytically,  or  cause 
cortical  stem  and  root  rot  or  vascular 
wilts.  F.  oxysporum  includes  sapro- 
phytes as  well  as  about  twenty-five 
named  forms  that  are  highly  specific  in 

1  Department  of  Plant  Pathology,  University 
of  California,  Berkeley;  unpublished  data. 


[  256  ] 


Fig.  124.  Effect  of  several  isolates  of  Rhizoctonia  solani  on  pepper  seedlings.  Seeds  were  sown 
in  treated  soil  and  immediately  inoculated  at  the  lower  end  with  equal  amounts  of  the  fungus 
isolate.  The  area  inside  the  white  line  is  the  zone  of  preemergence  damping-off.  Photos  taken 
18  days  after  planting.  Left  to  right,  top  row:  uninoculated;  inoculated  with  a  gladiolus  isolate 
(isolates  from  tung,  soybean,  cotton,  Dieffenbachia,  and  alfalfa  responded  in  the  same  way); 
inoculated  with  a  mild  isolate  from  pepper.  Left  to  right,  bottom  row:  inoculated  with  a  poin- 
settia  isolate;  inoculated  with  an  isolate  from  morning-glory  (an  isolate  from  lima  bean  re- 
sponded in  the  same  way);  inoculated  with  a  virulent  isolate  from  pepper.  The  mild  pepper 
isolate  caused  post-  but  no  preemergence  damping-off,  while  the  poinsettia  isolate  caused 
pre-  but  almost  no  postemergence  damping-off.  The  virulent  pepper  isolate  caused  both  the  pre- 
and  postemergence  phases. 


their  ability  to  invade  the  vascular  sys- 
tem of  plants.  These  forms  are  so  spe- 
cific that  one  of  them  will  attack  only 
certain  varieties  of  a  given  crop  species, 
a  fact  utilized  in  controlling  them 
through  resistance.  Thus,  the  form  which 
attacks  aster  is  limited  to  that  crop  and 
to  certain  varieties  of  it  as  well.  Oc- 
casionally a  form   appears  that  is  able 


to  attack  a  crop  variety  (for  example,  of 
tomato  or  pea)  previously  resistant  to 
the  Fusarium  wilt.  In  such  cases  this 
may  account  for  the  apparent  breakdown 
of  resistance,  because  in  reality  two  dif- 
ferent diseases  are  present.  As  in  Rhizoc- 
tonia, isolates  of  any  one  of  these  forms 
may  vary  widely  in  the  severity  of  dis- 
ease they  produce  in  a  given  susceptible 


[257] 


host  variety.  When  a  grower  is  indif- 
ferent about  introducing  a  Fusarium 
wilt  to  his  fields,  he  is  ignoring  the  dis- 
ease potentialities.  For  example,  he  may 
not  presently  be  raising  asters  and  thus 
be  unconcerned  about  the  dumping  of 
infested  aster  refuse  or  soil  on  his  land. 
Many  years  later  he  or  someone  else  may 
wish  to  plant  asters  in  these  fields,  and 
find  that  by  the  second  year  of  the  at- 
tempt, the  residual  fungus  has  built  up 
and  increased  disease  losses  to  a  ruinous 
extent. 

Some  growers  assume  that  all  water 
molds  are  alike  and  that  they  occur 
everywhere.  However,  some  strains  of 
Pythium  debaryanum  are  said  to  induce 
100  per  cent  root  rot  of  spruce  seedlings, 
while  other  strains  under  the  same  en- 
vironmental conditions  are  purely  sapro- 
phytic. Roth  and  Riker  (1943)  found 
that  the  damping-off  of  red  pine  varied 
from  36  to  87  per  cent  under  a  given  en- 
vironment, again  according  to  the  strain 
of  the  fungus  used.  It  is  now  clear  that 
the  worst  disease  problem  of  both 
heather  and  avocado  in  California  is 
root  rot  caused  by  Phytophthora  cin- 
namomi.  Although  water  molds  as  a 
group  may  be  generally  distributed,  this 
one  is  not  widespread  in  California  even 
yet,  and  those  plantings  suffering  from 
it  usually  can  trace  their  infestation  back 
to  the  nursery  source  of  the  stock.  There 
is  also  evidence  for  some  biological  spe- 
cialization within  species  of  this  genus. 
According  to  Tucker  (1931),  Phytoph- 
thora capsici  and  P.  parasitica  f.  nico- 
tianae  are  the  only  Phytophthora  species 
attacking  pepper  and  tobacco,  respec- 
tively. # 

Thielaviopsis  basicola  commonly 
causes  black  root  rot,  stem  decay,  or 
graft  failure  on  ornamentals;  it  has  a 
host  range  of  over  120  species  in  30 
families.  It  is  known  to  be  comprised  of 
races  which  cause  varying  amounts  of 
disease  on  some  crops  while  on  other 
crops  they  may  not  be  pathogenic  at  all. 
Isolates    from    poinsettia,    Primula    ob- 


conica,  cyclamen,  and  tobacco,  for  ex- 
ample, were  reported  in  some  inocula- 
tion experiments  as  most  pathogenic  to 
the  host  from  which  they  were  originally 
isolated,  while  the  reciprocal  inocula- 
tions yielded  less,  or  in  some  cases  no 
disease.  In  other  experiments  isolates 
from  tobacco  were  more  virulent  on 
Primula,  but  less  virulent  on  cotton 
seedlings  than  isolates  from  Primula 
itself. 

Bacteria 

Variability  in  virulence  has  also  been 
noted  in  the  bacterial  pathogens  that 
cause  fire  blight  and  bacterial  fasciation. 

Crown  gall  has  been  shown  to  have 
some  degree  of  host  specialization.  Iso- 
lates from  marguerite  daisy  will  produce 
medium-size  galls  on  tomato  and  rasp- 
berry, but  rarely  do  so  on  apple.  Rasp- 
berry and  probably  loganberry  isolates 
are  pathogenic  to  tomato  and  apple  but 
not  to  marguerite,  while  some  isolates 
from  apple  do  not  seem  to  be  pathogenic 
to  other  hosts. 

Nematodes 

There  are  also  host-restricted  races  in 
plant-parasitic  nematodes.  In  the  stem  or 
bulb  nematode,  Ditylenchus  dipsaci, 
some  of  the  "races"  are  comparatively 
unspecialized  and  are  parasitic  on  a 
wide  range  of  hosts;  other  "races"  are 
more  specialized  and  able  to  attack  only 
a  few  hosts,  while  still  others  may  sur- 
vive only  on  one  or  two  hosts.  The 
"races"  attacking  hyacinth  and  narcissus 
bulbs  fall  into  this  last  category.  Ap- 
parently nematodes  attacking  narcissus 
cannot  attack  hyacinth,  although  they 
may  attack  onions.  Recent  work  has 
shown  that  some  of  these  "races"  are 
actually  distinct  nematodes  (species). 
This  situation  further  illustrates  the 
danger  in  assuming  that  two  pathogens 
are  identical  because  they  are  presently 
called  by  the  same  name. 

In  what  is  generally  called  "the  root- 
knot   disease,"   as   though   caused   by   a 


[  258  ] 


single  nematode  species,  it  is  now  recog- 
nized that  several  are  actually  involved, 
and  that  they  differ  in  host  range.  Shalil 
peach  rootstock  in  some  areas  has  some- 
times "lost  resistance"  to  the  nematode 
when  attacked  by  a  different  population 
of  what  was  then  considered  as  one 
species.  Now  we  know  that  there  are 
three  species  of  root-knot  nematode  com- 
monly found  in  California,  only  one  of 
which  attacks  Shalil  rootstock.  Popula- 
tions from  Philodendron  sp.  are  able  to 
multiply  on  Persian  clover,  but  nema- 
todes from  Pothos  aureus  are  not.  Ac- 
cording to  the  species  or  race  of  the 
nematode  present,  some  plants  are  at- 
tacked in  one  locale  but  not  in  others.  It 
has  even  been  suggested  that  populations 
of  root-knot  nematode  can  be  identified 
on  the  basis  of  whether  or  not  they  attack 
peanuts,  pepper,  watermelon,  and  Lyco- 
persicon  peruvianum,  and  by  the  reaction 
of  snapdragons  to  them. 

Environmental  Response 

Not  only  may  different  strains  of  a 
given  microorganism  exhibit  differences 
in  respect  to  host  range  and  severity  of 
attack,  but  they  may  exhibit  differences 
in  response  to  physical  and  physiological 
factors  of  the  environment.  In  industry 
this  characteristic  of  physiological  vari- 
ation has  been  utilized  to  obtain  strains 
which  are  more  efficient  in  doing  specific 
jobs,  such  as  antibiotic  production, 
alcoholic  fermentations,  and  production 
of  dairy  goods,  as  well  as  an  array  of 
many  organic  compounds. 

Comparatively  little  is  known  about 
the  interactions  of  parasitic  organisms 
with  other  soil  microorganisms  and  with 
the  host,  and  of  the  effects  of  fungicides, 
soil  atmosphere,  light,  nutrient  or  vita- 
min deficiencies,  or  root  secretions  upon 
them.  Our  knowledge  in  this  field,  how- 
ever, is  rapidly  expanding. 

Temperature  relations 

It  has  been  reported  that  strains  of 
Rhizoctonia    vary    in    temperature    re- 


quirements for  disease  production.  The 
optimum  temperature  may  vary  from 
59°  to  95°  F.  When  two  strains  which 
differ  in  their  response  to  temperature 
exist  together  in  the  soil,  disease  will 
occur  over  a  wider  soil-temperature 
range  than  if  either  is  present  alone. 

Soil  depth  and  carbon 
dioxide  content 

Work  in  progress  by  the  author  in- 
dicates that  strains  of  Rhizoctonia  solani 
differ  widely  in  their  tolerance  to  the 
concentrations  of  carbon  dioxide  found 
in  the  soil.  The  fast-growing  aerial  iso- 
lates are  relatively  intolerant  of  carbon 
dioxide  and  therefore  probably  are 
unable  to  compete  successfully  with 
other  organisms  underground,  where  the 
carbon  dioxide  often  is  100  times  the 
atmospheric  concentrations.  Isolates 
found  at  or  near  the  soil  surface  are 
more  tolerant  than  aerial  strains,  but  are 
not  as  tolerant  as  some  found  attacking 
roots  3  to  18  inches  below  the  surface. 
This  subterranean  type  is  thus  able  to 
escape  competition  with  organisms 
found  at  the  soil  surface.  This  type  has 
become  increasingly  common  in  south- 
ern California  in  recent  years,  perhaps 
being  spread  with  planting  stock.  These 
relatively  tolerant  strains  can  more 
easily  grow  in  soils  which  have  poor 
aeration,  as  well  as  at  greater  depths  in 
the  soil,  andean  thus  parasitize  roots  that 
might  otherwise  escape  infection. 

Growers  can  with  safety  only  assume 
that  two  organisms  will  prove  different 
in  disease  potentialities  in  the  field,  re- 
gardless of  similarity  of  names  applied 
to  them  or  to  the  diseases  they  cause 
(fig.  124).  Evaluation  of  differences  in 
their  disease  potential  in  advance,  like 
the  determination  of  their  proper  name, 
is  a  specialist's  job.  From  the  growers' 
standpoint  each  strain  of  a  fungus  should 
be  considered  as  causing  a  distinct  dis- 
ease, in  that  the  introduction  of  a  new 
and  different  strain  may  increase  disease 
losses. 


[259] 


THE  INOCULUM  POTENTIAL 


Even  if  the  strains  of  a  pathogen  in- 
troduced into  the  soil  were  the  same  as 
those  already  present,  it  would  still  be 
unwise  knowingly  to  carry  in  more  of 
them  with  infested  planting  stock.  Such 
a  practice  usually  builds  up  the  amount 
of  the  organism  (inoculum  potential)  in 
the  soil,  and  would  also  distribute  it 
more  uniformly  through  the  field.  The 
inoculum  potential  is  important,  because 
the  disease  symptoms  which  we  see  are 
often  the  result  of  not  one  but  many 
attacks  on  the  plant  by  the  parasite. 
Naturally,  the  more  places  the  roots  are 
injured  the  more  will  be  the  loss  of 
normal  root  functions.  Usually  a  higher 
inoculum  potential  increases  the  inci- 
dence and  severity  of  the  disease. 


If  a  soil  is  only  slightly  infested  with 
the  aster-wilt  Fusarium,  there  usually 
will  be  only  a  little  disease  if  the  soil 
temperature  remains  below  60°  F,  but  at 
75°  to  80°  the  losses  will  be  severe.  In 
heavily  infested  soil,  severe  losses  will 
occur  even  below  60°.  Thus,  in  a  lightly 
infested  soil  a  grower  can  usually  escape 
severe  losses  in  coastal  California,  but  if 
he  builds  up  the  inoculum  he  will  be 
unable  to  produce  a  crop  in  any  season. 
It  has  also  been  found  that  if  a  soil  is 
heavily  infested  with  Rhizoctonia  solani, 
it  is  not  possible  to  control  damping-off 
by  the  dilute  formaldehyde  soil  treat- 
ment method  (Sec.  11),  or  by  coating 
the  seed  with  protective  fungicides; 
either  method  is  fairly  effective  in  lightly 
infested  soil. 


INFECTED  SEED  AND  STOCK 


Another  point  to  be  considered  is  that 
a  given  strain  of  a  fungus  introduced  on 
or  in  the  planting  materials  may  cause 
greater  and  more  rapid  disease  losses 
than  a  more  virulent  one  in  the  soil.  Be- 
cause the  fungus  is  already  in  the  tissue, 
or  is  so  situated  as  to  infect  the  host 
quickly,  the  rapidity  and  severity  of  loss 
may  be  more  or  less  independent  of  the 
amount  of  the  organism  present  in  the 
soil. 

It  has  been  stated  that  plant  materials 
have  been  the  source  of  some  90  per  cent 
of  the  plant  diseases  and  insect  pests 
which  have  come  to  us  from  other 
countries.  At  least  fourteen  pathogens  are 
carried  by  tomato  seed;  fourteen  by  iris 
rhizomes;  and  eight  by  pepper  seed. 

It  is  very  often  true  that  the  strains 
of  an  organism  carried  with  the  seed- 
ling or  on  the  seed,  bulbs,  root  divisions, 
or  other  plant  parts  are  particularly 
virulent   to  it.  Those  strains  best  able  to 


attack  the  host  do  so,  and  build  up  a 
high  inoculum  potential,  and  are  there- 
fore most  likely  to  be  carried  over  with 
the  stock.  For  example,  the  Rhizoctonia 
strains  stimulated  to  high  activity  in 
pepper-seed  fields,  because  of  the  pres- 
ence of  this  host,  are  also  most  likely  to 
cause  rot  of  the  fruits  in  contact  with 
the  soil  there.  Such  fruit  decay  leads  to 
infection  of  the  seed.  The  end  result  is, 
then,  that  pepper  seed  often  carries 
strains  of  R.  solani  highly  virulent  to 
pepper,  and  that,  when  these  seeds  are 
planted,  the  strains  are  established  in  the 
seedbed  and  new  planting.  One  of  the 
most  dangerous  features  of  the  transmis- 
sion of  organisms  with  seed  or  vegetative 
parts  is  that  the  constant  association  of 
the  virulent  strains  of  a  pathogen  and  its 
host  is  thus  assured.  Some  organisms 
such  as  the  oak  root  fungus,  Armillaria 
mellea,  cannot  easily  attack  vigorous 
growing    plants   directly    from    the   soil. 


r  260  i 


They  can  do  so,  however,  by  means  of 
rootlike  masses  of  mycelium  (rhizo- 
morphs)  that  grow  out  from  a  previously 
infected  plant  part.  In  this  case  the  intro- 
duction of  infected  plants  may  provide 
an  especially  effective  center  for  the 
spread  of  the  fungus.  Many  other  fungi 
undoubtedly  are  in  the  same  way  pro- 


vided with  an  effective  focus  for  spread 
in  a  planting.  Using  infected  stock  may 
more  than  nullify  any  benefit  gained 
from  soil  treatment.  For  these  reasons  the 
operation  of  an  isolation  house,  through 
which  all  new  plant  material  passes  be- 
fore being  planted  in  production  areas, 
is  a  sound  idea  (Sec.  13). 


LONGEVITY  IN  SOIL 


Many  of  these  organisms  are  able  to 
survive  indefinitely  in  the  soil.  There- 
fore, introductions  made  today  may  not 
be  evident  until  many  years  later  when 
a  proper  host  plant  is  cultivated.  An 
example  of  this  is  afforded  in  the 
branched  broomrape,  Orobanche  ra- 
mosa,  a  higher  plant  parasitic  on  roots, 
which  appears  to  have  been  introduced 
into  Alameda  County,  California,  on 
nursery  stock,  a  nonhost.  Subsequent 
cultivation  of  a  host,  tomato,  on  the 
surrounding  land  has  given  rise  to  the 
disease  which  in  recent  years  has 
reached  serious  proportions.  This  type 
of  situation  can  be  avoided  if  the  nur- 
series steam  the  soil  in  which  their  stock 
is  grown. 


Bacterial  blight  of  stock  carries  over 
for  1  year  in  the  soil  and  bacterial  stem 
rot  of  geranium  for  about  3  months. 
Organisms  of  this  type  do  not  persist  in 
the  soil  for  long  periods  in  the  absence 
of  their  hosts  because  they  apparently 
are  unable  to  compete  successfully  with 
other  soil  organisms.  On  the  other  hand, 
Rhizoctonia,  water  molds,  nematodes, 
Verticillium,  and  wilt  fusaria  can  exist 
for  many  years  saprophytically,  on  weed 
hosts,  or  as  resistant  structures  in  the 
soil.  These  facts  must  be  taken  into  con- 
sideration when  evaluating  disease-con- 
trol programs,  and  growers  should  be 
aware  of  the  far-reaching  dangers  in- 
volved when  dealing  with  pathogens 
capable  of  existing  many  years  in  soil. 


MIXED  INFECTIONS 


In  some  instances  disease  is  the  result 
of  attack  of  not  one,  but  two  or  more 
organisms.  In  these  cases  the  losses  may 
be  greater  than  the  injuries  from  each 
working  alone.  In  Fusarium  wilt  of  cot- 
ton, the  association  of  the  sting  nema- 
tode, Belonolaimus  gracilis,  with  the 
fungus  will  cause  losses  from  wilt  even 
in  supposedly  resistant  cotton  varieties. 
In  the  absence  of  the  nematode,  the 
fungus  is  able  to  cause  much  less  disease 
on  susceptible  varieties  and  does  not 
damage    resistant    varieties    at    all.    A 


similar  situation  prevails  in  the  inci- 
dence of  the  tobacco  black  shank  disease 
caused  by  Phytophthora  parasitica  f. 
nicotianae  in  association  with  root-knot 
nematode.  If  growers  introduce  either 
the  nematode  or  the  fungus  in  a  field  in 
which  the  other  part  of  the  complex  is 
present,  increased  disease  results.  This 
places  an  added  responsibility  on  the 
grower:  to  avoid  introducing  into  his 
growing  areas  a  pathogen  that  may  prove 
to  be  an  aggravating  agency  of  a  dis- 
ease complex. 


[261] 


OBLIGATION  OF  THE  NURSERY 


It  is  always  potentially  dangerous, 
and  often  an  immediate  risk,  to  intro- 
duce a  plant  pathogen  into  a  nursery  or 
field  regardless  of  whether  it  is  believed 
to  be  present  there  already.  It  is  probable 
that  the  greater  knowledge  of  tomorrow 
will  show  that  we  have  suffered  grave 
disease  losses  in  our  crops  because  of 
today's  thoughtless  spread  of  organisms 


believed  to  be  "already  present".  There- 
fore, the  nurseryman  has  a  special 
obligation  to  produce  stock  free  of  dis- 
ease organisms,  and  to  sell  only  those 
planting  media  (soil,  leaf  mold,  manure) 
that  are  also  free  of  such  pathogens.  To 
answer  the  initial  question:  There  is  no 
such  thing  as  a  safe  soil-infesting  disease 
organism  ! 


[262] 


SECTION 


Grower  Experience 
with  the  U.  C.  System 


R.  H.  Sciaroni 
J.  W.  Huffman 


Bedding  plants 

Vegetable  plants 

Pot  and  foliage  plants 

Can-grown  woody  plants 

Bench  and  bed  crops 

Cymbidiums  in  beds 

Landscape  application  of  the  U.C.  system 

General  experience 


HE  implications  and  explanations  of 
the  U.  C.  system  have  been  presented  in 
preceding  sections,  and  its  application 
and  mechanization  (Sec.  17)  in  several 
types  of  California  nurseries  remain  to 
be  reported.  To  illustrate  how  the  sys- 
tem is  being  adapted  to  varying  condi- 
tions, this  section  describes  the  expe- 
riences and  practices  of  twelve  growers 
of  bedding  plants,  pot  plants,  foliage 
plants,  vegetable  transplants  for  field  use, 
benched  flower  crops,  cymbidiums,  and 
can-grown  woody  stock  in  five  counties. 
These  changes  have  been  slow  to  come 
about,  and  were  in  the  majority  of  nur- 
series brought  about  only  when  tradition 
and  seemingly  standardized  practices 
were  broken.  Those  who  changed  to  the 
U.  C.  system  have  found  that  the  tech- 


nical assistance  of  a  well-trained  person 
has  reduced  errors  and  eased  the  transi- 
tion. 

Numerous  crops  (snapdragon,  carna- 
tion, stock,  calla,  delphinium,  Esther 
Read  and  Majestic  daisies,  violet,  gladi- 
olus) were  being  successfully  grown  in 
coastal  southern  and  central  California 
fields  of  the  same  fine  sandy  soil  that  is 
used  in  the  mix.  This  gave  additional 
confidence  in  the  U.  C.  system  and 
offered  the  possibility  of  transferring  the 
benefits  of  this  soil  to  pot  plant  and 
bench  culture. 

The  transition  began  in  commercial 
nurseries  in  southern  California  in  1943, 
but  became  general  after  1950.  In  the 
San  Francisco  Bay  area  apparently  no 
establishments    used    the    U.    C.    system 


[263] 


prior  to  1953;  instead  they  employed  the 
native  clay  loam  soils.  One  large  green- 
house establishment  in  central  California 
selected  the  location  of  its  new  range  in 
southern  California  on  the  basis  of  the 


presence  of  the  customary  clay  soil, 
ignoring  areas  of  the  fine  sand.  This 
range  is  now  hauling  in  for  its  green- 
house benches  the  soil  that  had  earlier 
been  rejected. 


BEDDING  PLANTS 


According  to  Mr.  Jack  L.  Mather, 
Manager  of  the  former  Bedding  Plant 
Advisory  Board,  California  State  De- 
partment of  Agriculture,  about  80  per 
cent  of  the  bedding-plant  growers  in 
southern  California  were  using  some 
type  of  light  soil  mix  in  1952-53.  By 
contrast,  many  growers  in  central  Cali- 
fornia are  still  using  clay  soils. 

Nursery  A . . . 

is  a  small,  well-established  bedding-plant 
concern  in  central  California.  This  was 
purchased  by  a  young  man  with  little 
prior  growing  experience,  although  he 
had  sold  retail  stock.  He  was  told  by  the 
people  from  whom  he  purchased  the 
nursery  and  by  other  local  growers  that 
some  crops  (alyssum,  phlox,  verbena, 
peppers,  and  eggplant)  could  not  be 
grown  in  that  climate.  Because  farm  ad- 
visors and  agricultural  inspectors  em- 
phasized the  importance  of  producing 
healthy  stock,  he  fumigated  his  soil  with 
methyl  bromide  to  rid  it  of  disease 
organisms  and  weeds.  Because  plant 
growth  was  still  poor,  he  sought  further 
assistance,  learning  that  the  nutritional 
and  salinity  levels  of  manures  and  leaf 
mold  varied  widely  and  were  therefore 
unreliable.  He  adopted  a  U.  C.-type  soil 
mix,  and  has  since  been  able  to  grow 
crops  very  successfully,  including  those 
which  could  not  previously  be  produced. 
II i^  intelligent  and  practical  approach, 
coupled  with  the  use  of  the  soil  mixes 
and  of  disease  control,  have  produced 
outstanding  results. 

Nursery  B  . . . 

is  a  large  establishment  in  southern 
California    thai    produces    flatted    stock 


for  field  planting  by  vegetable  and  cut- 
flower  growers  and  seedsmen,  as  well  as 
for  retail  sale.  Several  variations  of  the 
U.  C.  system  have  been  used  since  the 
nursery  was  started  in  1944.  The  soil  has 
consistently  been  steamed,  and  healthy 
planting  stock  or  heat-treated  seed  used. 
Weeding  of  flats  has  been  eliminated, 
this  benefit  alone  almost  paying  for  the 
cost  of  steaming.  The  nursery  is  fully 
mechanized  and,  therefore,  uses  a  mini- 
mum labor  force. 

During  a  period  when  only  peppers 
were  grown,  seed  was  machine-sown  in 
flats  which  were  stacked  until  germina- 
tion had  occurred.  This  saved  about  10 
to  14  days  of  growing  time  in  the  glass- 
house, and  an  additional  10  to  14  days 
were  saved  because  the  seedlings  were 
not  set  back  by  transplanting. 

Because  of  the  large  variety  of  crops 
presently  grown,  seedlings  are  trans- 
planted from  seed  flats  rather  than  the 
seed  sown  in  place. 

This  successful  nursery  is  probably 
the  oldest  user  of  the  U.  C.  system.  Be- 
cause damping-off  has  never  been  a  prob- 
lem here,  specialist  growers  have  exten- 
sively used  their  services  for  starting 
plants. 

Nursery  C  .  . . 

is  a  large  southern  California  bedding 
and  vegetable  plant  producer.  In  1948 
the  grower  was  attempting  to  maintain 
many  different  compost  soil  mixes  to  suit 
the  supposed  needs  of  the  many  dif- 
ferent varieties  of  plants  he  was  growing. 
He  had,  for  example,  a  petunia  mix, 
tomato  mix,  pansy  mix,  and  others.  His 
stock    piles    were    large    and    scattered, 


r  264  ] 


making  the  job  of  mixing  difficult  and 
expensive. 

Soil  sterilization  was  not  practiced 
because  these  organic  mixes  broke  down 
when  they  were  steamed  and  released 
toxic  amounts  of  soluble  salts.  Losses 
from  soil-borne  diseases  were  stagger- 
ing, the  only  control  practice  being  that 
of  reduced  watering. 

A  light  soil  mix  seemed  out  of  the 
question,  for  it  was  thought  to  be  costly 
and  inconvenient  because  of  the  neces- 
sity of  soil  hauling.  Most  important  of 
all  it  was  contrary  to  the  generally  ac- 
cepted beliefs  that  had  been  followed  in 
40  years  of  nursery  business. 

In  1949.  economic  factors  led  to  a 
break  in  the  tradition,  and  the  conver- 
sion to  a  L.  C.-type  soil  mix  got  under 
way.  A  small  concrete  mixer  was  used 
for  the  first  trials.  Results  were  so  out- 


standing that  a  large  concrete  transit 
mixer  was  purchased,  and  mechaniza- 
tion was  begun.  The  soil  storage  area 
has  been  reduced  from  IV2  or  2  acres  to 
about  1  -3  acre.  Two  or  three  men  are  now 
preparing  as  many  as  2,000  flats  per 
day,  a  job  that  formerly  required  eight 
or  ten  men.  Soil-borne  diseases,  except 
for  an  occasional  chance  recontamina- 
tion,  are  unknown.  X\  eeding  has  been 
eliminated.  All  jobs  have  been  standard- 
ized and  are  well  performed  with  semi- 
skilled labor  under  intelligent  super- 
vision. 

This  nursery  is  now  a  leader  in 
mechanization  and  one  of  the  strongest 
advocates  of  the  L.  C.-type  soil  mixes. 
The  owner  often  states.  "Each  step  leads 
to  the  next,  and  rapidly  pays  for  itself: 
the  only  barrier  to  the  light  soil  system 
is  tradition." 


VEGETABLE  PLANTS 


A  large  volume  of  celery  seedlings  is 
grown  under  glass  in  southern  California 
for  field  planting  in  areas  where  mosaic 
is  controlled  by  a  celery-free  period. 
Pepper,  eggplant,  and  tomato  are  also 
started  under  glass  for  special  purposes, 
sometimes  in  large  volume. 

Nursery  D  . . . 

is  a  large  producer  of  celery  seedlings  in 
Los  Angeles  County.  Tests  were  con- 
ducted by  the  Agricultural  Extension 
Service  in  this  nursery  in  1952-53,  com- 
paring a  treated  L.  C.-type  soil  mix  with 
their  conventional  untreated  composted 
soil. 

The  compost  consisted  of  %  decom- 
posed manure,  ^  black  peat,  and  1a  a 
soil-sand  blend,  and  had  been  composted 
for  a  year  or  more.  Decomposition  was 
not  uniform  because  of  differences  in 
temperature,  moisture,  amount  of  straw 
contained  in  the  manure,  and  other  fac- 


tors, evident  both  within  a  given  batch 
and  between  batches  and  seasons.  By 
contrast,  the  L .  C.-type  mix  required  no 
prior  preparation,  and  the  mix  could  be 
prepared  for  planting  immediately  upon 
delivery  of  materials.  Storage  space  for 
compost  piles  was  thus  saved.  Fertilizer 
top  dressing  of  the  seedlings  growing  in 
compost  cost  SO. 90  to  SI. 10  per  1.550 
flats,  whereas  application  of  calcium 
nitrate  i1^  oz.  per  gal.  on  a  10-day 
schedule)  to  the  L.  C.  mix  cost  only 
SO. 35.  an  additional  saving. 

It  was  found  in  four  tests  that  the 
plants  grown  in  the  L.  C.  mix  were 
salable  5.  6.  9.  and  11  days  sooner  than 
by  the  old  method.  Time  saved  in  this 
way  may  mean  that  an  extra  crop  can 
be  grown  during  the  busy  season.  In 
these  instances  the  differences  probably 
were  due  both  to  damping-off  and  exces- 
sive salts  in  the  compost.  Conductance 
1  Sec.  4 1  of  compost  initially,  and  after  2 


[265] 


weeks,  for  the  above  series  was:  6.4, 
dropping  to  3.7;  10.9,  dropping  to  5.4; 
8.3,  dropping  to  5.8;  12.1,  dropping  to 
6.9.  The  conductance  of  the  U.  C.-type 
mix  ranged  from  2.2  to  2.6  initially  and 
1.7  to  1.9  after  2  weeks. 

In  the  compost  the  root  systems  were 
poorly  developed  and  top  growth  was 
very  irregular  (height  varying  by  1  to 
1%  in.  in  a  single  flat) ,  whereas  plants 
in  the  U.  C.-type  mix  were  uniform  in 
size  (maximum  variation  %  in.)  and 
color,  and  were  larger. 

Production  in  the  compost  was  com- 
plicated, requiring  an  experienced  per- 
son to  water  the  flats,  shifting  from 
heavy  leaching  to  light  applications  as 
the   problem   changed   from   salinity   to 


damping-off  (fig.  35).  Plant  growth  was 
undoubtedly  depressed,  in  turn,  by  this 
practice,  since  celery  is  a  "wet  crop". 
With  the  U.  C.-type  mix,  a  regular  irriga- 
tion schedule  was  set  up  that  maintained 
proper  soil  moisture,  since  neither 
salinity  nor  damping-off  was  a  problem. 
The  seedlings  were  transplanted  to  com- 
mercial fields,  where  it  was  consistently 
found  that  those  grown  in  the  mix  re- 
mained green  and  started  growth  more 
quickly  than  those  grown  in  the  compost. 
Despite  the  demonstrated  effectiveness 
of  the  system  in  this  nursery,  the  nursery 
management  considered  that  it  was 
easier  to  continue  in  the  old  method  and 
too  much  trouble  to  change!  His  com- 
petitors, however,  did  make  the  change. 


POT  AND  FOLIAGE  PLANTS 


There  is  a  large  business  in  raising 
pot  plants  both  for  retail  sale  in  Califor- 
nia and  for  shipping  out  of  state.  Foliage 
plants  have  perhaps  the  fastest  expand- 
ing market  of  any  florist  crop  today,  and 
the  demand  is  largely  filled  from  Cali- 
fornia and  Florida.  Pot  plants  are  grown 
throughout  the  state,  but  there  tends  to 
be  a  concentration  of  foliage  crops  in 
southern  and  central  California. 

Nursery  E  . . . 

is  an  important  producer  of  pot  and 
foliage  plants  in  central  California.  The 
owners  formerly  used  multiple  soil 
mixes,  sometimes  changing  the  type  for 
each  crop,  or  for  a  given  crop  each  year, 
in  search  of  something  better.  In  1953 
they  changed  over  to  the  U.  C.  system 
of  soil  mixes  and  treatment.  This  has 
been  so  successfully  applied  that  mech- 
anization has  been  adopted  extensively; 
the  details  of  some  of  these  methods  are 
presented  in  Section  1 7.  Because  of  the 
labor  saving  effected,  four  men  are  able 
to  do  the  work  that  formerly  required 
twelve  to  fourteen. 


Two  years  of  commercial  experience 
has  demonstrated  that  practically  all 
types  of  foliage  and  blooming  pot  plants 
grow  very  well  under  the  U.  C.  system. 
The  cost  of  soil  preparation  has  been 
greatly  reduced,  and  certainty  of  results 
tremendously  increased.  The  results  have 
been  at  least  as  good  as  before,  and  in 
most  cases  superior. 

The  changeover  introduced  some 
problems,  however.  Poinsettias  planted 
at  the  regular  time  grew  so  rapidly  that 
they  were  far  too  large  for  the  average 
market.  The  following  season  this  was 
easily  corrected  by  delayed  planting  and 
double  pinching,  a  saving  in  time  and 
space  then  being  realized.  This  nursery 
is  a  successful  advocate  of  the  U.  C. 
system. 

Nursery  F 

This  southern  California  foliage-plant 
grower  began  operations  in  1948.  It  was 
soon  found  that  disease  losses  from  soil 
organisms  constituted  the  principal 
production    problem.    The    losses    from 


[266] 


Rhizoctonia  were  often  complete,  even 
though  the  foliage  was  kept  dry. 

He  tried  to  obtain  clean  stock  from 
various  sources  without  success,  and 
was  finally  forced  to  produce  it  himself. 
The  success  attained  with  Fittonia  ver- 
schaffeltii  var.  argyroneura  under  the 
U.  C.  system  illustrates  the  effectiveness 
of  the  methods  employed. 

The  planting  stock  was  treated  in  hot 
water  by  the  methods  explained  in  Sec- 
tion 13,  and  then  grown  in  individual 
pots  of  steamed  soil  similar  to  a  U.  C- 
type  mix.  The  plants  were  grown  to  a 
height  of  6  inches  under  conditions  of 
general  sanitation  and  without  ever  wet- 
ting the  foliage  before  cuttings  were 
taken.  The  cuttings  were  dipped  in 
Parzate  and  rooted  in  steamed  sand.  By 
the  time  this  procedure  had  been  fol- 
lowed for  three  generations,  adequate 
clean  mother  blocks  had  been  estab- 
lished. They  have  since  been  carefully 
maintained  and  have  yielded  consistentlv 
healthy  stock. 

At  first  the  cuttings  were  rooted  in 
sand  and  planted  in  small  pots,  taking 
8  to  10  weeks.  Now  the  cuttings  are 
planted  directly  in  small  pots  of  U.  C. 
mix   C    (50   per  cent  peat).   Automatic 


misting  of  cuttings  is  practiced  without 
loss  from  diseases  caused  by  Rhizoctonia 
or  water  molds.  Production  of  the  fin- 
ished plant  ready  for  sale  now  requires 
only  5  weeks.  Over  a  quarter  million 
young  plants  are  now  raised  annually 
in  scheduled  production  by  this  method. 
Similar  success  has  been  obtained 
with  Peperomia  obtusifolia  var.  varie- 
gata,  Peperomia  sandersii,  Pellionia  pul- 
chra,  Nephthytis  sp.,  Diefjenbachia  picta, 
D.  bausei,  and  Hedera  sp.  (Glacier  ivy) 
by  following  the  recommended  proce- 
dures for  eliminating  the  diseases. 

Nursery  G  . . . 

is  a  small  producer  of  foliage  plants, 
particularly  Philodendron  and  Dieffen- 
bachia,  in  central  California.  For  many 
vears  clav  soils  mixed  with  leaf  mold 

J  J 

and  manure  were  used.  Growth  was  ir- 
regular and  consistent  losses  were  ex- 
perienced because  of  overfertilization 
and  poor  drainage.  Fortunately  this 
nursery  is  located  on  a  Colma  fine  sand 
deposit.  The  grower  switched  all  of  his 
foliage  plant  operation  to  a  I.  C.-type 
mix  in  1953.  and  the  above  problems 
were  solved. 


CAN-GROWN  WOODY  PLANTS 


A  large  volume  of  nursery  stock  is 
grown  in  cans  (ranging  from  1  to  5 
gallons'  capacity)  in  California,  under 
lath  or  glasshouses,  or  outdoors.  This 
stock  is  sold  largely  for  home  planting. 

Nursery  H  . . . 

a  well-established  wholesale  nurserv  in 
central  California,  had  been  using  soils 
of  various  types,  mainly  clay.  They 
changed  to  a  U.  C.-type  soil  mix  and 
used  methyl  bromide  fumigation,  obtain- 
ing 25  to  50  per  cent  increased  growth 


over  their  standard  procedure  in  the  first 
season.  The  soil  did  not  shrink  away 
from  the  can  when  irrigation  was  de- 
layed as  did  the  clav.  making  for  easier 
and  more  effective  watering. 

After  part  of  a  season  of  successful 
growing,  it  was  decided  to  try  using  rice 
hulls,  an  inexpensive  and  abundant  or- 
ganic material  in  the  area.  The  mix  was 
altered  by  omitting  potassium,  which 
rice  hulls  supplied.  The  growth  has  been 
excellent,  containers  are  lighter  in 
weight,  and  cost  of  the  mix  has  been 
reduced. 


[267] 


BENCH  AND  BED  CROPS 


The  flower  crops  grown  in  raised 
benches  and  ground  beds  have  enormous 
value  in  California,  among  the  more  im- 
portant being  roses,  carnations,  chrysan- 
themums, and  gardenias.  Because  these 
plants  are  large  and  deep-rooted  and  do 
well  in  garden  soils,  it  would  be  expected 
that  the  benefits  from  using  a  U.  C.-type 
mix  would  be  less  than  with  the  foregoing 
crops. 

Nursery  I . . . 

grows  gardenias  in  raised  benches  in 
southern  California.  Using  a  soil  mix 
composed  largely  of  clay  and  peat,  they 
were  having  trouble  with  chlorosis  and 
death  of  plants.  The  soil  steaming  was 
only  partially  effective,  and  salinity  from 
overfertilization  and  a  poor  water  sup- 
ply created  an  additional  problem.  They 
tried  a  planting  in  U.  C.  mix  C  (50  per 
cent  peat) .  Uniform,  effective  steaming 
was  obtained,  with  little  or  no  evidence 
of  disease  in  the  .2%  years  of  the  test. 
The  plants  have  grown  very  well,  with 
greatly  increased  production.  The  use  of 
an  iron  chelate  in  the  winter  (2  oz.  per 
100  sq.  ft.)  completely  controls  normal 
cool-weather  chlorosis.  All  of  this  firm's 
plantings  are  now  in  this  mix.  The  rather 
poor-quality  water  is  no  longer  a  hazard. 
In  fact,  they  have  expanded  into  the  field 
of  foliage-plant  growing,  and  now  use 
the  same  mix  for  stock  beds  and  potted 
plants. 


Nursery  J  . . . 

is  a  carnation  grower  in  central  Califor- 
nia. On  the  basis  of  comparative  trials  of 
a  U.  C.-type  mix  in  raised  beds  as 
against  his  standard  clay  loam,  he  has 
shifted  to  the  light  mix  for  his  entire 
culture.  Several  outstanding  benefits 
have  been  observed  at  this  nursery.  Car- 
nations in  the  U.  C.  mix  showed  a  very 
low  percentage  of  calyx  "splitting"  as 
compared  to  those  grown  in  clay  loam. 
In  addition,  the  frequency  of  irrigation 
for  the  clay  loam  beds  was  almost  twice 
that  for  the  U.  C.  mix.  Further,  the  clay 
loam  upon  drying  developed  deep  cracks 
which  caused  severe  root  shearing;  this 
was  not  evident  with  the  U.  C.-type  mix. 

Nursery  K  . . . 

is  a  rose  grower  in  central  California. 
The  U.  C.-type  soil  mix  was  tested  in 
several  raised  beds  in  a  glasshouse.  The 
production  was  as  good  as  in  his  usual 
soil,  a  clay  loam  that  had  been  in  the 
benches  for  twenty  years.  Because  of  re- 
peated steaming  and  additions  of  peat 
during  this  period  the  soil  had  been 
brought  into  a  good  physical  structure. 
If  the  soil  should  have  to  be  changed  in 
the  benches  for  some  reason,  the  nursery 
plans  to  use  a  U.  C.-type  mix.  The  initial 
physical  structure  of  a  U.  C.-type  mix 
equals  that  attained  by  clay  soil  after 
several  years'  improvement  through 
organic  additions. 


CYMBIDIUMS  IN  BEDS 


Since   World   War   II    the   growth   of  Nursery  L .. . 

cymbidiurns  under  glass  or  lath  has  be-  raises   cymbidiums    in    ground    beds   in 

come  important,  particularly  in  southern  southern    California.    The    original    soil 

California.  mix  was  completely  organic,  consisting 

[268] 


of  leaf  mold,  manures,  and  bean  straw. 
The  soil  mix  was  placed  in  beds  12 
inches  deep  on  top  of  an  adobe  soil. 
Within  2  years  or  less  the  beds  shrank  to 
a  depth  of  approximately  6  inches,  owing 
to  decomposition  of  the  organic  mate- 
rials. The  nutritional  level  varied  widely 
throughout  the  nursery;  some  beds  were 
overfertilized,  others  were  underferti- 
lized.  Because  the  nutritional  levels  were 
unpredictable,  it  was  impossible  to  set  up 
fertilizer  applications  on  a  regular  sched- 
ule. The  over-all  result  was  irregular 
plant  growth  and  production. 

To  correct  this  problem  the  nursery- 
man changed  to  a  soil  mix  consisting  of 
fine  sand,  peat  moss,  and  pine  shavings, 


CAUTION: 

Many 

of 

the  i 

:hemicals 

mentioned 

in  this 

manual 

are 

poi- 

sonous   and 

may 

be 

harmful. 

The 

user  should 

carefu 

lly 

Follow  the 

pre- 

cautions  on 

the   1 

abe 

Is  of 

the 

con- 

tainers. 

with  known  nutrient  leveis.  The  applica- 
tion of  fertilizer  has  been  standardized 
and  placed  on  a  simple,  regular  schedule. 
The  volume  of  the  soil  in  the  beds  has 
remained  relatively  constant.  The  over- 
all results  are  reflected  in  uniform  plant 
growth  and  production  and  a  simplifica- 
tion of  management  practices.  Cost  of 
preparing  beds  has  been  reduced. 


LANDSCAPE  APPLICATION  OF  THE  U.  C.  SYSTEM 


A  large  race  track  in  southern  Califor- 
nia had  a  history  of  high  weeding  costs 
and  expensive  replacement  of  plants 
killed  by  disease.  To  overcome  this 
yearly  loss  the  infield  area  of  approxi- 
mately 10  acres  was  treated  with  methyl 


bromide  (fig.  125)  and  subsequently 
planted  with  disease-free  stock  pro- 
duced under  the  U.  C.  system.  Prior  to 
the  field  fumigation,  even  though  dis- 
ease-free stock  was  used,  as  many  as 
1,500  new  plants  were  needed  each  week 


Fig.  125.  Left,  methyl  bromide  fumigation  of  field  soil  in  southern  California  for  elimination 
of  pathogens  and  weeds.  This  is  a  field  adaptation  of  the  technique  developed  for  glasshouse 
soil  treatment.  Right,  the  same  field  2  months  after  planting  with  seedling  pansies  which  had 
been  grown  in  treated  nursery  soil.  This  illustrates  the  advantage  of  using  healthy  plants  in  clean 
soil;  a  more  vigorous  planting  results,  and  the  soil  is  not  reinfested  by  the  planting  stock. 


to  replace  those  killed  by  damping-off 
fungi.  After  fumigation,  only  400  plants 
were  replaced  in  a  4-month  season,  and 
most  of  these  were  mechanically  or 
chemically  injured.  This  illustrates  the 
necessity  of  both  soil  treatment  and 
clean  plants  in  disease  control  (fig. 
125).  As  another  benefit  from  soil  treat- 
ment, weed  control  was  reduced  from 
$60  per  acre  to  a  minor  figure. 

A  landscape  use  of  the  U.  C.  system 
was    the    replacing    of    heavy    disease- 


infested  soil  in  confined  beds  with  a 
fumigated  U.  C.-type  mix.  The  disease 
control  achieved  was  similar  to  that  de- 
scribed for  the  infield  planting.  Plants 
grown  in  a  treated  sand-peat  mix  were 
two  to  four  times  as  large  as  the  same 
stock  grown  in  the  treated  natural  loam 
of  the  infield.  The  use  of  a  U.  C.-type 
soil  mix  in  places  where  large  beds  are 
to  be  filled  with  hauled  soil  seems  to  be 
worthy  of  wider  trial  in  public  or  private 
plantings. 


GENERAL  EXPERIENCE 


The  results  of  grower  experience  with 
the  several  aspects  of  the  U.  C.  system 
have  shown  that: 

1.  It  is  uniquely  adapted  to  mecha- 
nization. 

2.  It  provides  a  continuous  supply  of 
a  uniform  growing  medium. 

3.  It  permits  the  use  of  fewer  and  less 
experienced  laborers.  These,  how- 
ever, should  understand  the  "why" 
as  well  as  the  "how"  of  what  they 
do,  and  should  be  directed  by  well- 
trained  supervisors. 

4.  It  provides  a  means  of  avoiding 
salinity  difficulties. 

5.  The  soil  provides  good  aeration  and 
water  drainage  for  root  develop- 
ment. 


6.  The  disease  problems  are  essentially 
eliminated  by  the  treatment  of  soil 
and  containers,  the  use  of  healthy 
planting  stock,  and  careful  culture 
to  maintain  them  in  that  condition. 

7.  The  cost  of  weeding  is  eliminated. 

8.  Post-steaming  toxicity  of  soil  is 
avoided. 

9.  Tedious  composting  procedures  are 
eliminated. 

10.  A  single  basic  mix  replaces  the 
many  formerly  used. 

11.  Materials  are  employed  that  are 
easily  and  cheaply  obtained. 

12.  It  enables  the  production  of  plants 
that  are  more  uniform,  healthier, 
and  larger,  at  lower  cost,  more  re- 
liably, and  faster  than  before. 


[270] 


SECTION 


m 


Mechanization 
and  the  U.C  System 


J.  W.  Huffman 


R.  H.  Sciaroni 


Mechanizing  an  old  nursery 

Planning  for  mechanization 

Stages  in  the  flow  of  materials 

Watering  and  fertilizing  in  the  glasshouse 


General  comments 


0 


ne  OF  the  major  advantages  of  the      tion    with    the    Agricultural    Extension 


U.  C.  system  for  the  nursery  is  the  ease 
with  which  this  type  of  production  may 
be  mechanized.  Increased  labor  cost, 
higher  taxes,  and  real-estate  subdivision 
have  made  nurserymen  anxious  to  attain 
greater  efficiency  (Sec.  2).  Residential 
development  has  forced  the  discontinu- 
ance of  composting  manure  piles,  be- 
cause of  their  odor  and  fly  problems. 
The  Agricultural  Extension  Service 
realized  that  changes  would  have  to  be 
made  if  urban  nurserymen  were  to  con- 
tinue in  business.  The  greatest  potential 
contribution  seemed  to  lie  in  standard- 
ization of  soil  mixes  and  mechanization 
through  the  U.  C.  system. 

During  the  developmental  period  of 
the  U.  C.  system,  perhaps  partly  because 
of  it,  intensive  mechanization  was  begun 
in  a  few  California  nurseries.  In  coopera- 


Service  and  Experiment  Station,  they 
adapted  ideas  from  materials-handling 
equipment  in  canneries,  assembly  lines, 
sand  and  gravel  operations,  concrete- 
mixing  plants,  and  other  well-engineered 
installations.  These  pioneering  growers 
exhibited  ingenuity  and  imagination  in 
devising  equipment  for  their  needs  in 
existing  nurseries. 

The  methods  and  equipment  de- 
veloped have  greatly  reduced  labor  re- 
quirements and  enabled  the  substitution 
of  semiskilled  for  scarce  highly  skilled 
labor  in  many  jobs.  Nursery  mechaniza- 
tion is  still  developing,  and  further  im- 
provements will  certainly  be  made. 
Since,  however,  there  is  such  a  pressing 
need  for  greater  efficiency  in  the  nursery 
industry,  it  is  desirable  that  the  tech- 
niques thus  far  developed  be  presented 
here. 


[271] 


SOIL  TREATED   IN   CONTAINERS 

Constituents  of  soil  mixture  clumped  in  piles  or  bins  on  delivery 

I 

Skip  loader 

I 

Soil  blended  and  moisture  added  in  concrete  mixer 

I 

Dumped  direct  or  conveyer  belt 

ci   .        "     c\\  , Untreated  flats  or  pots 

Flat  or  can  filler      ' — - — — — 

Manual 

Stacked  on  pallets 

I 

Forklift  tractor 


Steam  treatments: 


Piles  of  flats  or  cans  covered 

with  tarpaulin  (5). 

Horizontal  chamber  with  hood  (10). 

Horizontal  steam  vault  (6). 

Autoclave  (9). 


Chemical  treatments: 


Piles  of  flats  or  cans 

covered  with  plastic  sheet 

and  treated  with  methyl 

bromide  orchloropicrin 

aerosol. 


Forklift  tractor 
Piles  unstacked  by  hand 

I 

Conveyer  belts  or  rollers 


Planted  or  seeded  by  hand 


Machine  planted  or  seeded 


Conveyer  belts  or  rollers 

I 

Growing  area 

Fig.  126  (both  pages).  Diagrams  of  vaiious  methods  for  mechanization  of  thirteen  ways  to 
treat  soil  with  steam  or  chemicals  in  commercial  California  nursery  practice.  Numbers  refer  to 
equipment  types  (Sec.   10).  (From  a  chart  by  K.  F.  Baker.) 


[272] 


SOIL   TREATED   IN   BULK 

Constituents  of  soil  mixture  dumped  in  piles  or  bins  on  delivery 

I 
Skip  loader 
4< 
Soil  blended  and  moisture  added  in  concrete  mixer 

Dumped  direct  or  conveyer  belt 


V 

/ 

Mobile 

Mobile 

bin 

steam 

types 
i  \ 

box  (4) 

Pulled 

oy  tractor 

\ 

\ 

Bin 

(2) 

Dumped 

durr 

iped 

direct 

Mobile  units 
pulled  by  tractor 

Stationary  steam 

box  (4);  may  be 

in  tandem 


Treated  with  chloro- 
picrin  in  movable 
or  stationary  bins 
or  boxes,  or  with 
methyl  bromide  in 
piles  covered  with 
plastic  sheet. 


Dumped  direct 
or  conveyer  belt 

I 

Flat  or  can  filler 

Manual 


Bin  and 
potting  bench  (3) 


*    Stacked  on  pallets 

Forklift   I  tractor 

Piles  unstacked  by  hand 

f— 

Conveyer  belts  or  rollers 


\r 


Horizontal  revolving  drum  with 

steam  injected  (28)  or  with 

blow  torch  (30,  31). 

Screw  type  with  injected  steam  (29). 


Flats  or 

pots  treated 

with  steam  or 

chemicals. 


Dumped  into  flats  or  cans,  or 
into  flat  or  can  filler. 


Manual 


Planted  or  seeded  by  hand 


Machine  planted  or  seeded 


Conveyer  belts  or  rollers 
Growing  area 


[273] 


MECHANIZING  AN  OLD  NURSERY 


While  the  greatest  benefit  from  mech- 
anization obviously  is  gained  in  a  nur- 
sery designed  for  it,  impressive  savings 
have  often  been  made  by  adoption  in 
existing  nurseries  of  many  of  the  proce- 
dures outlined  here.  No  matter  how 
small  or  poorly  designed  a  nursery  may 
be,  some  of  the  mechanized  methods  de- 
scribed will  prove  adaptable  and  profit- 
able. 

The  grower  should  study  the  flow 
diagrams  of  mechanization  in  nurseries 
(fig.  126),  the  summary  chart  of  types 
of  soil  steamers  (table  15),  and  the  text 
and  illustrations  in  this  section  concern- 
ing mechanization  in  California  nur- 
series. He  should  observe  the  practices 
in  several  well-mechanized  nurseries;  his 


farm  advisor  or  the  authors  can,  if  de- 
sired, suggest  some  to  visit.  From  these 
several  sources  profitable  ideas  adapt- 
able to  the  specific  nursery  will  be  ob- 
tained. 

It  will  be  found,  furthermore,  that 
many  of  these  methods  may  be  adopted 
independently  and  consecutively,  with- 
out major  upheaval  or  expense  (see 
"Aids  in  Adopting  the  U.  C.  System," 
p.  1).  Many  nurserymen  have  demon- 
strated that  the  U.  C.  system  can  be 
adopted  in  progressive  easy  stages.  It  is 
important,  however,  that  the  process  be 
continued  until  a  complete  program  is 
established,  rather  than  stopping  at 
some  intermediate  level  of  partial 
benefits. 


PLANNING  FOR  MECHANIZATION 


The  exact  manner  of  mechanization 
must  be  developed  for  each  nursery, 
preferably  before  it  is  built.  Because  of 
smog,  population  pressures,  and  tax 
rates,  many  California  nurseries  may  be 
forced  to  move  in  the  next  several  years, 


house  equipment)  might  well  be  em- 
ployed. Some  of  the  basic  mechanization 
methods  presently  used  in  a  few  nur- 
series are  outlined  here  and  presented 
in  chart  form  (fig.  126). 

Some  of  the  factors  which  should  be 


and  this  affords  an  excellent  opportunity      considered   in   planning   a   new  nursery 


to  properly  design  the  new  units  for 
efficient  management.  For  this  reason,  it 
is  suggested  that  the  central  ideas  of  the 
II.  C.  system  be  tested  now,  and  that 
thought  be  given  to  incorporating  these 
principles  into  any  new  construction. 

Some  of  the  books  on  mechanization 
of  materials  handling  (see  Appendix, 
References)  should  be  consulted  in  the 
initial  stages  of  planning  to  be  sure  that 
the  besl  modern  methods  are  considered. 

If  (he  unit  is  to  be  a  large  one,  the 
services  of  an  engineer  who  specializes 
in  materials  handling  or  in  the  design  of 
continuous  processes  I  for  example. 
assembly    lines,    cannery    and    packing- 


may  be  suggested.  Careful  attention 
should  be  given  to  the  soil-treatment 
method  and  facilities  to  be  used  (sees. 
8  through  11). 

Utilizing  a  natural  slope 

If  the  land  has  a  natural  slope  this 
may  be  utilized  by  placing  the  soil  piles 
at  the  top,  followed  progressively  down- 
hill by  the  mixing  equipment,  container- 
filling  equipment,  treatment  facilities, 
planting  operations,  and  the  growing 
facilities.  Gravity  can  then  be  made  to 
do  much  of  the  transportation  work,  by 


using  steel  rollers. 


[  274  ] 


Glasshouse   arrangement 
and  design 

Glasshouses  should  be  arranged  so 
that  they  branch  off  both  sides  of  a 
central  corridor  through  which  tem- 
porary steel  rollers  may  be  set  up,  lead- 
ing from  the  soil  area.  Plans  should  in- 
clude openings  for  steel  rollers  to  run 
into  each  house  through  the  end  wall, 
rather  than  the  doors.  Containers  may 
then  be  taken  almost  directly  to  their 
places  in  the  glasshouse  without  exces- 
sive lifting  or  carrying. 

The  width  of  the  glasshouse  aisles 
should  be  considered  in  terms  of  the 
equipment  to  be  pushed  through  them, 
and  vice  versa.  One  nursery  decreased 
the  labor  of  emptying  glasshouses  by 
designing  them  so  that  the  sides  were 
removable  down  to  bench  level,  to  per- 
mit the  removal  of  flats  through  the  sides 


of  the  houses.  Flats  were  moved  on  steel 
rollers  and  loaded  directly  onto  truck 
beds  (at  the  same  height)  for  transporta- 
tion to  the  area  where  the  plants  were 
hardened-off  before  sale. 

The  size  and  orientation  of  the 
benches  should  be  studied  for  greatest 
mechanization  potential.  If  bench  crops 
are  to  be  grown,  the  width  of  the  benches 
and  any  steam  pans  to  be  used  should  be 
the  same.  Structural  pipes  through  beds 
should  be  avoided  whenever  possible,  as 
they  invariably  increase  cost  of  steam 
treatment. 

Paving  of  yard 

The  area  around  all  houses  should  be 
paved  to  expedite  mechanization  and 
reduce  weed  growth,  from  which  insects, 
often  virus-carrying,  move  into  the 
glasshouses. 


STAGES  IN  THE  FLOW  OF  MATERIALS 


Processing  and  Stockpiling 
the  Materials 

Possible  sources  of  the  fine  sand  were 
discussed  in  Section  6.  A  statewide  sur- 
vey for  exact  sources  has  been  con- 
ducted,1 and  information  on  this  may  be 
obtained  through  your  local  farm  ad- 
visor. It  should  be  emphasized  that  the 
various  truckloads  of  the  fine  sand 
should  be  checked  on  delivery  for  uni- 
formity in  conforming  to  physical  (Sec. 
6)  and  salinity  (Sec.  4)  standards,  if 
there  is  any  reason  to  suspect  variability. 
To  facilitate  this,  it  is  well  to  order  this 
material  somewhat  in  advance  rather 
than  wait  until  it  is  actually  needed.  Be- 
cause of  the  storage  space  required,  it  is 
not  usual  to  stockpile  large  quantities. 
Since  no  composting  is  necessary  in  pre- 

^y  M.  H.  Kimball,  Ornamental  Horticul- 
turist, California  Agricultural  Extension  Serv- 
ice. 


paring  the  mix,  no  space  is  required  for 
this  process.  Usually  a  supply  sufficient 
for  2  to  3  weeks  is  kept  on  hand.  The 
Canadian  or  German  peat  moss  may  be 
obtained  from  your  local  horticultural 
supply  house. 

Both  of  these  ingredients  should  be 
stored  in  bins  under  a  roof,  preferably 
on  a  large  concrete  slab,  or  at  least  in  a 
well-drained  area.  This  structure  might 
well  be  large  enough  also  to  accommo- 
date the  mixing  operation.  There  would 
then  be  only  short  hauls  between  the  dif- 
ferent steps  of  this  procedure,  and  opera- 


CAUTION: 

Many 

of  the  < 

:hemicals 

mentioned 

in  this 

manual 

are 

poi- 

sonous   and 

may 

be    harmful. 

The 

user  should 

carefu 

Ily  follow  the 

pre- 

cautions  on 

the  1 

abels  of 

the 

con- 

tainers. 

[275] 


tions  could  continue  during  rains.  This 
building  should  be  separated  from  the 
soil-sterilizing  and  planting  facilities  in 
order  to  reduce  recontamination,  and 
should  be  located  for  greatest  conveni- 
ence in  carrying  the  soil  or  the  filled 
containers  to  the  soil-treatment  equip- 
ment and  glasshouses. 

Storage  of  Soil 

The  U.  C.  mixes  may  be  stored  in- 
definitely if  organic  nitrogen  has  not 
been  added,  but  should  not  be  held  more 
than  a  week  if  they  contain  more  than  a 
small  amount  of  such  material  (sees.  5 
and  7).  Soil  is  preferably  stored  before, 
rather  than  after  treatment  to  reduce  the 
recontamination  hazard.  Some  growers, 
however,  have  satisfactorily  stored 
treated  soil  in  a  tight  building  where 
wind-blown  soil  will  not  reach  it  (fig. 
128).  Such  a  structure  should  not  be 
near  the  mixing  operations.  The  build- 
ing should  be  humidified,  perhaps  by 
mist  nozzles,  to  reduce  drying  of  the  soil 
if  prolonged  storage  is  contemplated. 
Raising  humidity  to  prevent  drying  of 
flats  is  preferred  to  direct  watering, 
which  is  extremely  difficult  since  the 
flats  are  stacked  on  pallets.  Watering  be- 
fore use  also  has  a  tendency  to  compact 
the  flats  and  cause  the  transplanter  to  go 
through  an  unnecessary  step  of  loosen- 
ing the  soil  before  transplanting. 

Excellent  temporary  storage  is  pro- 
vided by  placing  a  tarpaulin  over  the 
stacked  flats.  The  flats  should  be  kept  out 
of  the  sun. 

Making  Flats 

In  one  large  nursery,  a  sufficient  num- 
ber of  flats  was  used  so  that  it  was 
economic  to  purchase  a  box-making 
machine  especially  designed  for  south- 
ern California  flats.  This  reduced  hand- 
construction  and  hence  costs.  Several 
smaller  nurseries  might  go  together  in 
purchasing  such  equipment,  or  a  port- 
able  unit  might  be  taken,  on  a  service 
basis,  from  one  nursery  to  another  to 
prepare  the  season's  supply. 


Preparing  the  Soil  Mix 

Equipment  needed 

The  presently  accepted  and  generally 
adopted  method  for  preparing  nursery 
soils  is  to  use  a  stationary  concrete 
mixer  for  about  a  10-minute  period. 
These  are  available,  new  or  used,  in 
various  sizes.  The  larger  sizes  have  a 
loading  apron  into  which  the  ingredients 
are  conveniently  dumped  by  a  skip- 
load  tractor.  Used  transit-type  mixers 
(figs.  127  through  130)  with  a  capacity 
of  about  5  cubic  yards  are  commonly 
utilized.  They  cost  $400  to  $800  and  must 
be  cleaned  of  cement,  reconditioned,  and 
painted  before  use.  Small  nurseries  use 
1-  to  2-cubic-yard  cement  mixers.  Several 
growers  use  two  mixers  to  reduce  the 
time  of  the  operation. 

Some  nurseries  have  tried  mixing 
their  soil  with  a  skip  loader  and  screens, 
but  have  found  this  unsatisfactory. 

An  important  benefit  of  the  U.  C. 
system  is  that  no  complicated  equipment 
is  required  for  breaking  up  clods.  // 
hard  lumps  of  soil  are  present  in  the  fine 
sand,  it  is  not  of  the  proper  type  (Sec. 
6).  Several  nurseries  in  central  Califor- 
nia have  each  recently  spent  $800  to 
$1,000  for  equipment  to  break  up  the 
lumps  in  the  clay  soil  before  mixing 
could  begin. 

The  only  equipment  used  for  prepar- 
ing the  ingredients  for  a  U.  C.-type  mix 
is  a  small  shredder  sometimes  employed 
for  breaking  up  the  bales  of  peat.  More 
frequently  the  peat  is  wetted  and  is 
broken  up  by  a  fork. 

If  the  soil  contains  debris  or  rocks  it 
may  be  screened  at  this  time  (rare),  or 
after  mixing  (see  below). 

Mixing 

The  fine  sand  and  moistened  shredded 
peat,  as  well  as  the  proper  weights  of 
the  desired  amendments  and  fertilizers 
(sees.  5,  6,  and  7)  are  dumped  into  the 
mixer  in  their  proper  relative  propor- 
tions. During  the  rotations  of  the  mixer 


[276] 


, 


.##&& 


Fig.  127.  General  view  of  a  bedding-plant  nursery,  showing  the  mechanization  of  soil  handling. 
Transit  mixers  (A)  mix  the  soil  ingredients,  which  are  then  conveyed  (B)  to  a  rotating  screen  (C) 
above  the  flat  filler  (D).  The  flats  are  carried  on  steel  rollers  (E)  under  the  filler,  and  are  stacked 
on  pallets  (F),  which  are  transported  by  the  fork-lift  (G)  to  the  steam  chamber. 


Fig.  128.  Another  view  of  the  same  soil-handling  operation.  The  steam  chamber  (H)  is  shown 
in  use.  If  soil  is  to  be  stored  temporarily,  it  is  moved  to  the  enclosed  building  (I)  by  the  fork-lift. 
The  peat  supply  and  empty  flats  are  shown  in  the  foreground,  the  fine  sand  is  to  the  immediate 
left  (not  shown). 


a  measured  amount  of  water  is  some- 
times added  to  bring  the  mix  to  a  uni- 
form desired  moisture  content  for  steam- 
ing and  planting  (Sec.  8) .  It  is  generally 
preferable,  however,  to  moisten  the  in- 
gredients before  mixing.  A  ribbon  mixer 
of  the  type  used  in  mixing  plaster  has 
proved  excellent  for  soil. 

If  adequate  storage  space  is  planned, 
the  nursery  may  use  the  soil-mixing  and 
container-filling  equipment  only  2  to  3 
days  a  week,  using  two  or  three  men. 
Note,  however,  that  the  mix  containing 
organic  nitrogen  should  not  be  stored 
for  longer  than  a  week  before  planting 
(sees.  5  and  7) . 

It  requires  about  30  minutes  to  han- 
dle a  load  in  concrete  mixers.  Thus  the 
capacity  in  cubic  yards,  multiplied  by  2, 
will  give  the  hourly  capacity. 

If  the  mix  is  not  to  be  screened,  it  may 
be  taken  from  the  mixer  directly  or  car- 
ried   by    a    conveyor    belt    or    skip-load 


tractor  (see  "Transporting  the  Soil,"  be- 
low) directly  to  the  flat  filler,  to  the 
treatment  equipment  (if  it  is  to  be 
treated  in  bulk),  or  to  a  combined  bin 
and  potting  bench  (Sec.  10). 

Screening 

When  the  soil  is  mixed  it  is  sometimes 
dumped  directly  onto  a  moving  belt  that 
conveys  it  to  a  revolving  circular  screen 
8  to  10  feet  high  (figs.  127,  128,  and 
130).  These  units  are  custom-made  for 
each  nursery  and  cost  about  $1,500.  The 
belt  width  is  about  16  inches,  and  the 
unit  is  driven  by  a  gasoline  engine  or 
electric  motor.  The  screen  is  3  to  4  feet 
in  diameter  and  5  to  6  feet  long.  Usually 
there  is  little  debris  in  the  ingredients 
and  this  procedure  is  done  as  much  to 
continue  mixing  as  to  get  rid  of  stones, 
sticks,  rubbish,  or  large  pieces  of  peat. 
The  mix  may  fall  through  the  screen  di- 
rectly into  the  hopper  of  the  flat  filler,  or 
into  a  pile. 


Fig.  129.  Method  of  filling  a  transit  mixer  by  means  of  a  skip  loader.  The  proportion  of  fine 
sand  and  moistened  peat  is  determined  by  the  number  of  loads  of  each.  The  tandem  arrange- 
ment of  mixers  assures  a  continuous  output  of  mixed  soil. 


Filling  the  Containers 

It  is  generally  desirable  to  place  the 
soil  in  the  containers  prior  to  treatment. 
Sometimes,  however,  the  sequence  may 
at  this  point  be  reversed.  The  soil  is  then 
treated  in  bulk  before  it  is  placed  in  con- 
tainers that  have  been  treated  separately. 
Treated  soil  should  never  be  placed  in 
untreated  containers. 

The  flat  filler  (fig.  130)  or  container 
fillers  (figs.  10  and  135)  vary  more  in 
construction  than  other  pieces  of  equip- 
ment used  in  the  mechanized  nursery.  In 
general  they  consist  of  a  wide-mouth 
tapered  hopper,  the  soil  flowing  through 
the  adjustable  opening  onto  a  variable- 
speed  rubber  belt.  The  soil  drops  from 
the  end  of  this  belt  into  a  flat  or  other 
container  below,  that  is  carried  on  an- 
other belt  moved  by  power  or  manually. 
The  width  of  the  lower  opening  of  the 
hopper  and  the  width  of  the  belts  may  be 
varied  for  different  types  of  containers. 


Flat  fillers 

Some  of  the  flat  fillers  use  an  auger 
gear  or  leveling  bar  to  level  the  soil  in 
the  flats.  Some  of  these  devices  will  han- 
dle 900  flats  per  hour.  One  man  is  re- 
quired to  place  the  flats  on  the  input 
conveyor  belt,  and  one  to  remove  the 
filled  flats  at  the  output.  Flats  are  usually 
stacked  on  hardwood  pallets  (figs.  127 
and  130)  (36  by  54  in.)  to  be  transported 
by  a  fork-lift  tractor  (fig.  127).  The 
price  of  this  type  of  equipment  is  about 
$1,000;  the  hardwood  pallets  are  about 
$2.75  apiece. 

This  step  may  be  difficult  to  mechanize 
if  flats  are  not  of  uniform  size  or  have 
wide  bottom  cracks.  Paper  liners  are 
sometimes  used  in  such  old  flats.  In 
southern  California  the  flats  are  uni- 
formly 18  by  18  by  3  inches;  those  in 
central  California  are  quite  variable  in 
size.  This  latter  situation  is,  of  course, 
an  obstruction  that  must  be  resolved  if 


Fig.  130.  Detail  of  screening  and  filling  operation,  showing  transit  mixer  (A),  conveyor  belt 
(B),  rotating  screen  (C),  flat  filler  (D),  and  steel  rollers  (E).  The  worker  is  removing  flats  from  the 
steel  rollers  and  placing  them  on  a  pallet.  Note  the  wood  separator  strips  (arrow),  which  are 
placed  between  layers  of  flats. 


economical     mechanization     is     to     be 
adopted. 

Mechanical  can  and  pot  fillers 

Mechanical  fillers  are  available  (see 
Appendix)  for  1-gallon  cans  and  pots 
(fig.  135)  and  for  2-  and  5-gallon  cans 
(fig.  10).  Soil  may  be  conveyed  to  this 
equipment  after  being  mixed,  and 
screened  and  treated  in  bulk;  the  filled 
containers  are  planted  at  once. 

Hand-filling 

Some  nurseries  fill  the  containers  by 
hand  at  the  point  of  mixing.  They  may 
then  be  piled  on  pallets  as  before  and 
carried  by  a  fork-lift  tractor.  However 
filling  may  be  done,  the  number  of  flats 
placed  on  the  pallet  will  depend  on  the 
size  of  the  tractor,  the  size  and  shape  of 
the  containers,  and  the  dimensions  of 
the  treatment  chamber. 

Mobile  bin  and  potting  bench 

In  one  nursery,  the  soil  from  the 
mixer  is  dumped  onto  a  mobile  bin  and 
potting  table  (Sec.  10,  type  3) .  This  flat- 
bed wagon  (fig.  132)  has  a  movable 
tongue  hitch,  and  can  easily  be  pulled 
by  a  light  truck  or  tractor.  Perforated 
pipe  is  permanently  mounted  in  the  bot- 
tom of  the  wagon  and  is  easily  con- 
nected to  the  generating  source  for 
steaming.  It  is  covered  by  a  tarpaulin 
during  this  process.  After  steaming  it  is 
pulled  into  the  potting  headhouse.  In  a 
few  hours  the  soil  is  cool,  and  one  side 
of  the  wagon  is  let  down  for  a  work 
table.  Potting  and  planting  are  done  di- 
rectly from  the  treated  pile.  The  soil  mix 
is  easily  worked  into  pots  around  bulbs 
or  rooted  plants. 

Treating  Soil  and  Containers 

The  details  have  already  been  given 
for  steam  treatment  (sees.  8,  9,  and  10) 
and  chemical  treatment  (Sec.  11)  of 
nursery  soils.  Equipment  has  been  de- 
scribed (Sec.  10)  for  steaming  of  the 
-oil.    Chemical    treatment    is    generally 


done  in  stacks  of  staggered  flats  or  on 
wooden  pallets.  In  either  case,  treatment 
operations  are  preferably  isolated  from 
those  of  soil  mixing  or  flat  filling  to  re- 
duce possible  recontamination. 

The  cost  of  materials  and  labor  for 
constructing  a  steam  chamber  (figs.  128 
and  131 )  varies  from  $250  to  $500,  ac- 
cording to  the  type  and  size  of  installa- 
tion. Marine  plywood  (waterproof  glue) 
sheets  %  inch  thick  and  4  by  10  feet  in 
size  should  be  used  for  sides,  doors,  and 
top.  Exterior  plywood  is  not  suitable  for 
this  purpose.  The  frame  is  constructed 
of  either  metal  or  2  by  4  lumber.  De- 
tails on  this  and  other  types  of  equipment 
are  given  in  Section  10. 

Transporting  the  Soil 

Untreated  soil 

Untreated  ingredients  are  most  eco- 
nomically transported  by  a  skip-load 
tractor  (figs.  126  and  129)  from  storage 
piles  into  the  soil  mixer.  From  there  the 
mix  may  again  be  transported  by  a  skip- 
loader  or  be  carried  by  a  moving  rubber 
conveyor  belt  to  the  flat  filler,  or  car- 
ried by  a  rubber  belt  to  the  revolving 
screen,  from  which  it  is  dumped  into  the 
flat  filler.  Alternatively,  the  mixed  soil 
may  be  taken  by  skip-loader  to  a  bulk 
steam  treater,  or  the  steamer  (if  mobile) 
may  be  brought  under  the  mixer  and 
screen  and  filled. 

Filled  containers 

Containers  are  usually  stacked  on 
wooden  pallets  and  carried  by  a  fork- 
lift  tractor  (figs.  127  and  131).  The  cost 
of  this  piece  of  equipment  is  variable, 
depending  on  the  size  of  the  unit;  a 
large  unit  with  pneumatic  tires  used  by 
some  nurseries  costs  $3,200  to  $3,500. 
Since  the  fork-lift  is  efficiently  used 
in  many  nursery  operations,  its  cost 
should  not  be  completely  charged  to 
soil  handling.  At  one  large  pot-plant 
nursery  the  fork-lift  mounted  at  the  rear 


[280] 


Fig.  131.  Loading  a  pallet  of  filled  flats  into  a 
wooden  vault  (type  6,  Sec.  10)  steam  chamber 
by  means  of  a  fork-lift.  The  vault  is  unloaded 
in  the  same  manner,  and  taken  by  fork-lift  to 
the  seeding  or  planting  operation. 


Fig.  132.  Mobile  bin  and  potting  bench  (type 
3,  Sec.  10),  showing  the  potting  operation.  The 
soil  is  dumped  into  the  bin  from  an  elevated 
transit  mixer  and  steamed  from  pipes  in  the 
bottom  of  the  wagon,  which  is  then  pulled  to 
the  potting  area.  With  this  type  of  equipment 
care  must  be  exercised  to  avoid  introducing 
pathogens  into  the  exposed  soil  mass  from  oc- 
casional diseased  propagating  material  during 
the  potting  operation. 


Fig.  133.  Bicycle-wheel  cart  used  for  trans- 
porting potted  stock  into  the  glasshouse  from 
the  potting  bench.  This  is  adaptable  to  glass- 
houses with  narrow  aisles  or  where  conveyor 
belts  are  not  practical. 


Fig.  134.  Rack  for  transporting  pots  to  a 
vault-type  steamer,  or  to  be  covered  with  a 
tarp  and  steamed  or  treated  with  methyl  bro- 
mide. Unit  is  transported  by  a  fork-lift  tractor. 

of  the  tractor  is  used  to  transport  steri- 
lized clay  pots  (fig.  134). 

The  containers  may  be  taken  from  the 
treatment  chamber  by  the  fork-lift  trac- 
tor  on   the    same   pallets,   to   the   place 


where  they  will  be  planted.  They  may, 
however,  be  carried  either  on  steel  rollers 
or  a  conveyor  belt.  After  planting  they 
generally  are  taken  on  portable  steel 
rollers  to  their  location  in  the  glass- 
house, sections  of  rollers  being  removed 
as  the  house  is  filled. 

A  special  cart  equipped  with  bicycle 
wheels  (fig.  133)  has  been  constructed 
for  use  with  the  mobile  bin  and  potting 
bench  mentioned  above.  The  cart  has  a 
low  center  of  gravity  for  good  balance, 
and  the  narrow  bed  facilitates  rapid 
transit  of  planted  pots  through  glass- 
house aisles. 

Planting 

Transplanting  seedlings 

Planting  is  an  extremely  varied  opera- 
tion. It  varies  from  transplanting  of 
seedlings  into  flats  by  hand,  an  opera- 
tion that  seems  to  defy  mechanization, 
to  planting  them  in  pots.  The  latter  op- 
eration has  been  mechanized  by  the 
Erdprinz  planter  in  Germany    (see  Ap- 


Fig.  135.  Automatic  can  or  pot  fillers  for  placing  soil  in  1-gal.  containers  and  forming  a  cen- 
tral depression  into  which  the  liner  is  planted.  (See  also  fig.  10.)  (Photo  courtesy  of  Oki  Nursery, 
Perkins,  California.) 

[  282  1 


pendix)  and  equipment  for  use  with  the 
U.  C.  mixes  has  been  developed  in  Cali- 
fornia (figs.  10  and  135). 

Machine-seeding 

Equipment  has  been  used  from  time 
to  time  for  spot-planting  the  seed  in  con- 
tainers. One  unit  was  used  for  several 
years  for  such  planting  of  pepper  seeds 
in  flats.  It  consisted  of  a  vacuum  plate 
that  fitted  the  inside  dimensions  of  the 
flat,  and  had  a  hole  drilled  where  each 
seed  was  desired.  This  plate  was  alter- 
nated between  a  tray  of  clean  seed  (with 
the  vacuum  on)  and  a  flat  (with  the 
vacuum  then  released).  It  was  possible 
to  seed  150  flats  per  hour  in  this  way. 
The  seed  and  soil  were  covered  with  tis- 
sue paper,  then  by  sterile  clean  sand,  and 
watered  generously.  Similar  planting 
plates  are  available  on  special  order 
(see  Appendix). 


Mechanical  seeders  of  this  type  will 
not  operate  efficiently  on  tiny  seed,  and 
attempts  to  evade  this  by  pelleting  the 
seed  to  larger  size  plunge  one  into  germi- 
nation difficulties.  In  one  series  of  tests 
by  P.  A.  Chandler  (unpublished  data), 
pelleted  seeds  of  Theodosia  Improved 
petunia  gave  best  germination  when  they 
were  placed  on  the  surface,  pressed 
lightly  into  the  soil,  and  then  sprinkled. 
All  other  methods  were  quite  inferior. 
The  germination  difficulties  and  dif- 
ferences in  seed  size,  vitality,  and  germi- 
nation time  have  all  indicated  a  dubious 
future  for  pelleting  of  fine  seed. 

An  experimental  mechanical  seeder 
recently  seen  may  solve  these  difficulties. 
Until  one  is  perfected,  fine-seeded  plants 
will  require  hand-transplanting.  For 
large-seeded  plants  machine-seeding  has 
for  several  years  been  an  accomplished 
fact. 


WATERING  AND  FERTILIZING  IN  THE  GLASSHOUSE 


In  some  cases  it  has  been  satisfactory 
to  water  flats  in  the  glasshouse  by  over- 
head sprinklers.  Because  of  the  excellent 
drainage  of  a  U.  C.-type  soil  mix  and 
freedom  from  damping-off  fungi,  there 


is  little  danger  from  applying  excess 
water.  Thus,  enough  water  can  be  ap- 
plied to  satisfactorily  wet  nearly  all  of 
the  flats,  and  the  few  that  need  more  can 
be  hand-watered.  This  procedure  saves 


Fig.  136.  Liquid-fertilizer  injector  for  accurately  diluting  nutrients  into  the  water  stream  during 
irrigation.  (Photo  courtesy  of  Smith  Precision  Products  Co.,  South  Pasadena.) 


[283] 


much  labor,  but  can  only  be  used  when 
a  whole  glasshouse  area  is  reasonably 
uniform. 

Some  growers  have  applied  fertilizer 
through  such  a  system.  One  grower  of 
Kentia  palms  successfully  fertilized  in 
this  way  for  several  years.  Another 
grower  of  pot  foliage  plants  has  also  ob- 
tained excellent  results,  using  a  fertilizer 
injector    (fig.    136)    in   the   water   line. 


Care  must  be  taken  to  flush  the  chemical 
from  the  pipes. 

A  better  procedure  is  probably  pro- 
vided by  hand  application  of  liquid 
fertilizer  by  means  of  special  injectors 
(fig.  136  and  Appendix)  which  operate 
through  the  watering  hose.  Fertilizers 
may  also  be  applied  to  the  surface  of 
soil  in  pots  in  the  dry  form  (sees.  5,  6, 
and  7) . 


GENERAL  COMMENTS 


The  total  cost  of  mechanizing  the  soil- 
mixing  process  is  about  $6,000  to 
$8,000,  according  to  the  size  of  the  nurs- 
ery. To  avoid  this  large  initial  invest- 
ment, most  nurserymen  with  establish- 
ments already  built,  develop  one  or  two 
steps  at  a  time,  beginning  with  the  ce- 
ment mixer.  Each  subsequent  operation 
is  developed  and  designed  for  the  spe- 
cific operation.  This  partly  explains  the 
wide  variation  in  types  of  equipment 
used  in  California. 


Results  from  mechanization  have  gen- 
erally been  outstanding  in  producing 
better  plants  at  lower  cost.  It  is  often 
found  that  two  to  three  men  working  2 
or  3  days  a  week  are  taking  care  of  all 
soil  mixing  in  a  nursery  that  formerly 
required  six  or  more  working  all  week 
for  this  operation.  Sometimes  the  saving 
is  even  greater. 

The  nursery  has  yet  to  be  built  that 
fully  utilizes  all  the  potentialities  of  the 
U.  C.  system.  Will  yours  do  this? 


[284] 


N  D  I  X 


REFERENCES 

Section  2 

California  State  Commission  of  Housing 

1954.  California  housing.  California  State  Dept.  Indus.  Relations,  Div.  Housing,  p.  13,  15. 
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Section  3 

Baker.  K.  F. 

1946.  Observations  on  some  Botrytis  diseases  in  California.  Plant  Dis.  Reptr.  30:  145-55. 

1947.  Seed  transmission  of  Rhizoctonia  solani  in  relation  to  control  of  seedling  damping-ofl. 
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1948.  Nursery  seedlings.  Improved  methods  for  production  possible  with  control  of  damping-off 
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Baker,  K.  F.,  and  R.  H.  Sciaroni 

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Beach,  W.  S. 

1949.  The  effects  of  excess  solutes,  temperature  and  moisture  upon  damping-off.  Pennsvlvania 
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Chitwood.  B.  G..  and  W.  Birchfield 

1956.    Nematodes,  their  kinds  and  characteristics.  Florida  State  Plant  Board  Bui.  2  (9)   :  1—49. 
Ellis.  D.  E..  and  R.  S.  Cox 

1951.    The  etiologv  and  control  of  lettuce  damping-off.  North  Carolina  Agr.  Exp.  Sta.  Tech. 
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Gravatt,  G.  F. 

1954.    Potential  danger  to  the  Persian  walnuts,  Douglas-fir,  and  Port  Orford  cedar  of  the  Pacific 
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HlLTABRAND.  W.  F. 

1951.    Soil  treatment  as  an  aid  to  the  pinto  tag  program.  Pacific  Coast  Nurseryman  10  i'8">  :  13. 
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1938.    Combating  damping-off.  New  York  (Geneva)  Agr.  Exp.  Sta.  Bui.  683:  1—16 
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1956.    Flooding  injury-  studied  by  approach-graft  and  split  root  system  techniques.  Amer  Jour. 
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Leach.  L.  D. 

1947.    Growth  rates  of  host  and  pathogen  as  factors  determining  the  severity  of  preemergence 
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McClure,  T.  T.,  and  W.  R.  Robbins 

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Steiner,  G. 

1953.  Plant  nematodes  the  grower  should  know.  Florida  State  Dept.  Agr.  Bui.  n.s.  131:  1-48. 
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1944.    The  root-knot  nematode.  California  Agr.  Exp.  Sta.  Cir.  330:  1-30. 

Section  4 

Baker,  K.  F.,  O.  A.  Matkin,  and  L.  H.  Davis 

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Baker,  K.  F.,  and  R.  H.  Sciaroni 

1952.    Diseases  of  major  floricultural  crops  in  California.  57  p.  California  State  Florists'  Assoc, 
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Bernstein,  L.,  M.  Fireman,  and  R,  C.  Reeve 

1955.  Control  of  salinity  in  the  Imperial  Valley,  California.  U.  S.  Dept.  Agr.,  Agr.  Res.  Serv. 
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Bernstein,  L.,  A.  J.  MacKenzie,  and  B.  A.  Krantz 

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Hickman,  D. 

1949.  Soluble  salts  rot  Saintpaulia  leaves.  Illinois  State  Florists'  Assoc.  Bui.  No.  Ill:  1. 
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1952.  Comparison  of  growth  of  tomato  plants  in  impervious  plastic  pots  and  porous  clay  pots. 
New  Zealand  Jour.  Sci.  Tech.,  Sec.  A,  34:  365  68. 

Lunt,  O.  R.,  and  M.  H.  Kimball 

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McCormick,  J.  A. 

1949.    Germination  failure  of  shallow-seeded  vegetable  crops.  California  Agr.  Ext.  Serv.,  Mon- 
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Pearson,  H.  E. 

1949.    Effect  of  waters  of  different  quality  on  some  ornamental  plants.  Amer.  Soc.  Hort.  Sci. 

Proc.  53:  532-42. 
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Richards,  L.  A.,  editor. 

1954.  Diagnosis  and  improvement  of  saline  and  alkali  soils.  U.  S.  Dept.  Agr.,  Agr.  Handbk. 
60:  1-160. 

Schoonover,  W.  R.,  S.  Wilhelm,  and  R.  H.  Sciaroni 

1952.    Testing  greenhouse  soils  for  soluble  salts.  California  Agr.  Ext.  Serv.  Pub.  6  p. 
Wilhelm,  S.,  and  H.  T.  Pyfrom 

1949.    Soluble  salt  injury  to  gardenia.  California  Agr.  3  (12)  :  5,  12. 

Sections  5  and  6 

Aldrich,  D.  G.,  and  J.  P.  Martin 

1952.    Effect  of  fumigation  on  some  chemical  properties  of  soils.  Soil  Sci.  73:  149-59. 
Allerton,  F.  W.,  and  A.  G.  Ray 

1954.    Complete  specification.  Improvements  in   agricultural  or  horticultural   composts.   Brit. 
Patent  Office  Pat.  Spec.  715,978.  3  p. 
Allison,  F.  E.,  and  M.  S.  Anderson 

1951.    The  use  of  sawdust  for  mulches  and  soil  improvement.  U.  S.  Dept.  Agr.  Cir.  891:  1-19. 
Vnderson,  M.  S.,  S.  F.  Blake,  and  A.  L.  Mehring 

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1948.    Partial  sterilization.  Soils  and  Fort.  11  (6)  :  357  60. 
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Bayer,  L.  D. 

1956.    Soil  physics.  3d  ed.  489  p.  John  Wiley  and  Sons,  Inc.,  New  York,  N.Y. 
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1952.    Principles  of  plant  physiology.  499  p.  See  specifically  p.  11-73.  W.  II.  Freeman  and  Co., 
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1936.    Directions  for  making  mechanical  analyses  of  soils  by  the  hydrometer  method.  Soil  Sri. 
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1952.    The   U.   C.   soil   mix  for  nurseries  in   California.   Pacific   Coast   Nurseryman    11    (1)  : 
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1952.    Factors  affecting  the  growth  of  sugar  cane.  Hawaii  Agr.  Exp.  Sta.  Tech.  Bui.  18:  1-90 
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1936.    The  comparative  moisture-absorbing  and  moisture-retaining  capacities  of  peat  and  soil 
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1935.    Distribution   of   roots   in   porous   and   nonporous   plant   containers.   Plant   Physiol.    10: 
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1932.    Value  of  peats  for  mineral  soil  improvement.  Boyce  Thompson  Inst.  Contrib.  4:  245-55. 

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1954.    A  chlorosis  of  citrus  produced  by  biuret  as  an  impurity  in  urea.  Florida  State  Hort.  Soc. 
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1952.  Mechanisms  of  ion  absorption  by  roots.  Ann.  Rev.  Plant  Physiol.  3:  189-206. 
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1956.    Effect  of  biuret  on  bean  plants  grown  in  different  soil  types.  Michigan  Agr.  Exp.  Sta. 
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1954.    Diagnosis  and  improvement  of  saline  and  alkali  soils.  U.  S.  Dept.  Agr.,  Agr.  Handbk. 
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|  2l\i\  | 


Section  7 

Bonner,  J. 

1950.    Plant  biochemistry.  537  p.  See  specifically  p.  222-44.  Academic  Press  Inc.,  New  York, 
N.Y. 
Bonner,  J.,  and  A.  W.  Galston 

1952.    Principles  of  plant  physiology.  499  p.  See  specifically  p.  253-84.  W.  H.  Freeman  and 
Co.,  San  Francisco,  Calif. 
Bi  rkhart,  L. 

1934.    Metabolism  of  etiolated  seedlings  as  affected  by  ammonium  nutrition.  Plant  Physiol.  9: 
351-58. 
Clark,  K.  G. 

1952.    Urea-form — new  nitrogen  fertilizer.  Crops  and  Soils  4  (8)  :  14-15. 
Davies,  J.  N. 

1954.    Steam  sterilization  studies.  Cheshunt  Exp.  Res.  Sta.  Ann.  Rept.  39:  47-53. 

1956.    Steam  sterilization  studies.  Cheshunt  Exp.  Res.  Sta.  Ann.  Rept.  40:  54-58. 
Davies,  J.  N.,  and  0.  Owen 

1952.    Steam  sterilization  studies.  Cheshunt  Exp.  Res.  Sta.  Ann.  Rept.  37:  67-78. 
Duisberg,  P.  C,  and  T.  F.  Buehrer 

1954.  Effect  of  ammonia  and  its  oxidation  products  on  rate  of  nitrification  and  plant  growth. 
Soil  Sci.  78:  37-49. 

Grogan,  R.  G.,  and  F.  W.  Zink 

1956a.  Fertilizer  injury  and  its  relationship  to  several  previously  described  diseases  of  lettuce. 

Phytopathology  46:  416-22. 
19566.  Fertilizer  injury  to  lettuce.  Damage  reproduced  by  application  of  toxic  concentrations 
of   inorganic   commercial    fertilizer   materials   or    animal   manure.    California   Agr.    10 
(12)  :5,  12,  16. 
Hewitt,  E.  J. 

1952.    Sand  and  water  culture  methods  used  in  the  study  of  plant  nutrition.  Commonwealth 
Bur.  Hort.  Plant.  Crops  Tech.  Comm.  22:  1-241. 
Klein,  R. 

1956.    Nitrification  in  steam-sterilized  soil.  Cheshunt  Exp.  Res.  Sta.  Ann.  Rept.  40:  29-32. 
Lawrence,  W.  J.  C,  and  J.  Newell 

1950.    Seed  and  potting  composts,  with  special  reference  to  steam  sterilization.  4th  ed.  166  p. 
George  Allen  and  Unwin  Ltd.,  London,  England. 
Long,  M.  I.  E.,  and  G.  W.  Winsor 

1956.    Urea-formaldehyde  compounds  as  nitrogenous  fertilizers.  Cheshunt  Exp.  Res.  Sta.  Ann. 
Rept.  40:  43-49. 
Lorenz,  O.  A. 

1956.    Aqua  and  anhydrous  ammonia,  good  sources  of  nitrogen  fertilizers  but  materials  should 
be  placed  well  away  from  plants  to  lessen  chance  of  injury.  California  Agr.  10  (11)  :  7. 
Owen,  O.,  and  J.  N.  Davies 

1950.    Steam  sterilization  studies.  Cheshunt  Exp.  Res.  Sta.  Ann.  Rept.  35:  63. 
Owen,  O.,  and  D.  W.  Rogers 

1947.  The  availability  of  nitrogen  in  some  organic  fertilisers.  Cheshunt  Exp.  Res.  Sta.  Ann. 
Rept.  32:  66-70. 

Owen,  O.,  and  G.  W.  Winsor 

1948.  Nitrification  studies.  Cheshunt  Exp.  Res.  Sta.  Ann.  Rept.  33:  69-80. 

1949.  Nitrification  studies.  Cheshunt  Exp.  Res.  Sta.  Ann.  Rept.  34:  62-70. 
Owen,  0.,  G.  W.  Winsor,  and  M.  I.  E.  Long 

1952.    The  properties  of  some  urea-formaldehyde  materials  in  relation  to  their  possible  use  as 
nitrogenous  fertilizers.  Jour.  Sci.  Food  Agr.  3:  531-41. 
Thiegs,  B.  J. 

1955.  Effect  of  soil  fumigation  on  nitrification.  Down  to  Earth  (Midland,  Mich.)  11  (lj  :  14—15. 
Tiedjens,  V.  A.,  and  W.  R.  Robbins 

1931.    The  use  of  ammonia  and  nitrate  nitrogen  by  certain  crop  plants.  New  Jersey  Agr.  Exp. 
Sta.  Bui.  526:  1-46. 


[289] 


Weissman,  G.  S. 

1951.  Nitrogen  metabolism  of  wheat  seedlings  as  influenced  by  the  ammonium :nitrate  ratio 
and  the  hydrogen-ion  concentration.  Amer.  Jour.  Bot.  38:  162-74. 

Wood,  J.  G. 

1953.    Nitrogen  metabolism  of  higher  plants.  Ann.  Rev.  Plant  Physiol.  4:  1-22. 

Sections  8  and  9 

Ball,  V. 

1953.    Modern  methods  of  steaming  soils.  Grower  Talks    (West  Chicago,  111.)    17    (4)  :   1-7; 

(5)  :  1-7. 
1956.    Soil    sterilizing.    Grower    Talks    (West    Chicago,    111.)     20    (1):    1-11;     (2):    16-22; 
(3)  :  15-20.  Also  in:  Ball  Red  Book,  ed.  9.  p.  26-48.  1957. 
Baver,  L.  D. 

1956.    Soil  physics.  3d  ed.  489  p.  John  Wiley  and  Sons,  Inc.,  New  York,  N.Y. 
Beachley,  K.  G. 

1937.    Combining    heat    and    formaldehyde    for    soil    treatment.    Pennsylvania   Agr.    Exp.    Sta. 
Bui.  348:  1-19. 
Bewley,  W.  F. 

1939,  1948.  Practical  soil  sterilization  with  special  reference  to  glasshouse  crops.  Brit.  Min.  Agr. 
Fisheries  Bui.  22:  1-29  (3d  ed.)  ;  1-22  (4th  ed.). 
Bosworth,  R.  C.  L. 

1952.  Heat  transfer  phenomena.  The  flow  of  heat  in  physical  systems.  211  p.  John  Wiley  and 
Sons,  Inc.,  New  York,  N.Y. 

Bouyoucos,  G.  J. 

1913.    An  investigation  of  soil  temperature  and  some  of  the  most  important  factors  influencing 

it.  Michigan  Agr.  Exp.  Sta.  Tech.  Bui.  17:  1-196. 
1915.    Effect    of   temperature    on    some   of   the   most    important    physical    processes   in    soils. 
Michigan  Agr.  Exp.  Sta.  Tech.  Bui.  22:  1-63. 
Brown,  C.  A.  C,  and  P.  Wakeford 

1947.    Electrical  soil  sterilization  by  immersion  heaters.  Brit.  Elect.  Allied  Indus.  Res.  Assoc. 
Tech.  Rept.  Ref.  W/T  14:  1-17. 

BUEHRER,  T.  F. 

1932.    The  movement  of  gases  through  the  soil  as  a  criterion  of  soil  structure.  Arizona  Agr. 
Exp.  Sta.  Tech.  Bui.  39:  1-57. 
Bunt,  A.  C. 

1954,  1955.  Steam  pressure  in  soil   sterilization.  I.  In   bins.  II.   Glasshouse  in  situ  sterilizing. 

Jour.  Hort.  Sci.  29:  89-97;  30:  43-55. 

1955.  Steam-air  mixture.  John  Innes  Hort.  Inst.  Ann.  Rept.  45:  28. 
Bunt,  A.  C,  and  W.  J.  C.  Lawrence 

1955.    Balanced  steaming  of  glasshouse  soil.  John  Innes  Hort.  Inst.   Lflt.    14:    1-7. 
Dimock,  A.  W.,  and  K.  Post 

1944.    An  efficient  labor-saving  method  of  steaming  soil.  New  York   (Cornell)    Agr.  Ext.  Bui. 
635:  1-7. 
Emerson,  P. 

1930.    Principles  of  soil  technology.  402  p.  Macmillan  Co.,  New  York,  N.Y. 
Fishenden,  M.,  and  O.  A.  Saunders 

1950.    An  introduction  to  heat  transfer.  205  p.  Oxford  Univ.  Press,  London,  England. 
Frank,  B. 

1888.    Ueber  den  Einfluss,  welchcn  das  Sterilisiren  des  Erdbodens  auf  die  Pflanzen-Entwicke- 
lung  ausiibt.  Dent.  Bot.  Gesellsch.  Ber.  6  ( General versammlungber.)  :  87-97. 
Hall,  C.  W. 

1955.    Which   fuel  for  your  boiler?  Michigan  Florist    (Lansing,  Mich.)    No.  292  :  24. 
Ho  \kk,  E.  R. 

1953.    Temperature  measurement  with  special  reference  to  frost,  steam  sterilization  and  glass- 
bouse  climates.  13th  Internatl.  Hort.  Cong.  Kept.  2:  843  53. 
Johnson,  J  \  mes 

1916.    Soil-steaming  for  disease  control.  Soil  Sci.  61:  83  91. 


[  290  | 


Keenan,  J.  H.,  and  F.  G.  Keyes 

1936.    Thermodynamic  properties  of  steam,  including   data  for  the  liquid  and   solid   phases. 
89  p.  John  Wiley  and  Sons,  Inc.,  New  York,  N.Y 
Kersten,  M.  S. 

1949.  Thermal  properties  of  soils.  Univ.  Minnesota  Engineering  Exp.  Sta.  Bui.  28:  1-227. 
Lawrence,  W.  J.  C. 

1956.  Soil  sterilization.  171  p.  George  Allen  and  Unwin  Ltd.,  London,  England. 
MacLean,  J.  D. 

1930.    Studies  of  heat  conduction  in  wood.  Results  of  steaming  green  round  southern  pine 
timbers.  Amer.  Wood-Preserv.  Assoc.  Proc.  26:  197-217. 
Morris,  L.  G. 

1954a.  The  steam  sterilising  of  soil.  Experiments  on  fine  soil.  Brit.  Nat'l  Inst.  Agr.  Engin.  Rept. 

14:  1-32. 
19546.  The  steam  sterilising  of  soil.  The  application  of  research  to  practice.  Brit.  Nat'l  Inst. 
Agr.  Engin.  Rept.  24:  1-22. 
Morris,  L.  G.,  and  F.  E.  Neale 

1957.  The  steam  sterilising  of  soil:  the  heating  of  deep  soil  and  the  surface.  Brit.  Nat'l  Inst. 
Agr.  Engin.  Tech.  Memo.  114.  (In  press.) 

Morris,  L.  G.,  and  K.  W.  Winspear 

1957.    The  steam  sterilising  of  lumps  of  soil.  Brit  Nat'l  Inst.  Agr.  Engin.  Tech.  Memo.  113. 
(In  press.) 
Newhall,  A.  G. 

1930.    Control  of  root-knot  nematode  in  greenhouses.  Ohio  Agr.  Exp.  Sta.  Bui.  451:  1-60. 
1940.    Experiments  with  new  electric  devices  for  pasteurizing  soils.  New  York  (Cornell)   Agr. 

Exp.  Sta.  Bui.  731:  1-38. 
1955.    Disinfestation  of  soil  by  heat,  flooding  and  fumigation.  Bot.  Rev.  21:  189-250 
Newhall,  A.  G.,  C.  Chupp,  and  C.  E.  F.  Guterman 

1940.    Soil  treatments  for  the  control  of  diseases  in  the  greenhouse  and  the  seedbed.  New  York 
(Cornell)  Agr.  Ext.  Bui.  217:  1-58. 
Patten,  H.  E. 

1909.    Heat  transference  in  soils.  U.  S.  Dept.  Agr.  Bur.  Soils  Bui.  59:  1-54. 
Penman,  H.  L. 

1940.    Gas  and  vapour  movements  in  the  soil.  II.  The  diffusion  of  carbon  dioxide  through  porous 
solids.  Jour.  Agr.  Sci.  30:  570-81. 
Perkins,  J.  J. 

1954.  Bacteriological  and  surgical  sterilization  by  heat.  In:  G.  F.  Reddish.  Antiseptics, 
disinfectants,  fungicides,  and  chemical  and  physical  sterilization,  p.  655-82.  Lea  and 
Febiger,  Philadelphia,  Penn. 

Precision  Scientific  Co. 

1955.  Dry  heat  sterilization.  Precision  Scientific  Co.  (Chicago,  111.)  Bui.  337:  1-15. 
Rudd,  W.  N. 

1893.    Killing  grubs  in  soil.  Amer.  Florist  (Chicago,  111.)  9  (278)  :  171. 
Schwarz,  A.  R.  VON 

1879.    Vergleichende  Versuche   iiber  die  physikalischen   Eigenschaften  verschiedener   Boden- 
arten.  Forsch.  Gebiete  Agr.-Phys.  2:  164-69. 
Senner,  A.  H. 

1934.    Application  of  steam  in  the  sterilization  of  soils.  U.  S.  Dept.  Agr.  Tech.  Bui.  443:  1-20. 
Stone,  G.  E.,  and  R.  E.  Smith 

1898.    Nematode  worms.  Massachusetts  Agr.  Exp.  Sta.  Bui.  55:  44-65. 
Stout,  G.  L.,  and  W.  F.  Hiltabrand 

1954.    Approved  treatment  and  handling  methods  for  vegetable  plants   grown  in   flats  to  be 
shipped   under   Intercounty   Nursery   Stock   Certificates.   California   Dept.   Agr.   Mimeo. 
3063-4.  1  p. 
Tavernetti,  J.  R. 

1950.  Heat  is  best  for  soil  sterilization.  Pacific  Coast  Nurseryman  9  (8)  :  14,  32. 
Togashi,  K. 

1949.  Biological  characters  of  plant  pathogens  temperature  relations.  478  p.  Meibundo  Co., 
Tokyo,  Japan. 


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Zimmerman,  0.  T.,  and  I.  Lavine 

1945.    Psychrometric    tables    and    charts.    162    p.    Industrial    Research    Service,    Dover,    New 
Hampshire. 

Section  10 

Anonymous 

1940.    Formaldehyde  and  steam  mixture  for  sterilizing  greenhouse  soil.  DuPont  Agr.  News 
Letter  8  (5)  :  71-72. 

1952.  Steam  sterilizing  soil.  Ohio  Florists  Assoc.  Bui.  No.  276:  2-3. 

1953.  Steam  soil  sterilizer  firm  names  national  distributor.  Pacific  Coast  Nurseryman  12  (5)  : 
46-47. 

1954.  Soil  handling.  Grower  Talks  (West  Chicago,  111.)  18  (4)  :  26. 

1955.  [Portable  steam  generator.]  Grower  Talks  (West  Chicago,  111.)  19  (8)  :  22. 

1956.  Mr.  Jackson  improves  his  steam  traveller.  The  Grower  (London)  46:  991,  993,  995. 

1957.  Growers  see  a  steam  plough  from  Holland.  The  Grower  (London)  47:  97. 
Ball,  G.  K. 

1942.    Sterilizing.  Why  and  how?  26  p.  Geo.  J.  Ball  Inc.,  West  Chicago,  111. 
Ball,  V. 

1953.  Modern  methods  of  steaming  soils.  Grower  Talks    (West  Chicago,  111.)    17    (4)   :  1-7; 

(5)  :  1-7. 

1954.  Steaming  aster  cloth  houses.  Grower  Talks  (West  Chicago,  111.)  17  (10)  :  18-19. 

1955.  Steam  rake  sterilizing.  Grower  Talks  (West  Chicago,  111.)  18  (10)  :  21-24. 
Beachley,  K.  G. 

1937.    Combining  heat  and  formaldehyde  for  soil  treatment.  Pennsylvania  Agr.  Exp.  Sta.  Bui. 
348:  1-19. 
Bewl'ey,  W.  F. 

1939.    Practical  soil  sterilization  with  special  reference  to  glasshouse  crops.  3d  ed.  Brit.  Min. 

Agr.  Fisheries  Bui.  22:  1-29. 
1948.    Practical  soil  sterilization  with  special  reference  to  glasshouse  crops.  4th  ed.  Brit.  Min. 
Agr.  Fisheries  Bui.  22:  1-22. 
Brown,  C.  A.  C,  and  P.  Wakeford 

1947.    Electrical  soil  sterilization  by  immersion  heaters.  Brit.  Elect.  Allied  Indus.  Res.  Assoc. 
Tech.  Rept.  W/T  14:  1-17. 
California  Department  of  Agriculture,  Bureau  of  Entomology  and  Plant  Quarantine 
1944.    An  inexpensive  soil  pasteurizing  equipment.  California  Dept.  Agr.  Nursery  Pest  Control 
Lflt.  A-l:  1-3. 
Canham,  A.  E. 

1951.    Electrode  soil  sterilizing.    (Preliminary  report.)    Brit.  Elect.  Allied  Indus.  Res.  Assoc. 
Tech.  Rept.  W/T  21:  1-11. 

COATES,  W. 

1954.  New  device  for  steam  sterilising  soil.  The  Grower  (London)  42  (26)  :  1247. 
Dimock,  A.  W.,  and  K.  Post 

1944.    An  efficient,  labor-saving  method  of  steaming  soil.  New  York  (Cornell)   Agr.  Ext.  Bui. 
635:  1-7. 
Fosler,  G.  M. 

1950.    An  efficient  steam  sterilizing  box.  Illinois  Agr.  Ext.  Serv.,  Rept.  from  Hort.,  Flower 
Growing  No.  9  :  1-2. 
Hansen,  A. 

1953-54.  Dampning.  Gartnertidende  18  (6)  :  1  p.;  19  (1)  :  1  p. 
Hardy  and  Dillon,  Inc. 

1953.    H  and  D  electric  heater  assembly  for  soil  sterilizers.  4  p.  Hardy  and  Dillon,  Inc.,  Boston, 
Mass. 
Johnson,  J. 

1930.    Steam  sterilization  of  soil  for  tobacco  and  other  crops.  U.  S.  Dept.  Agr.  Farmers'  Bui. 
1629:  1-13. 
Lawrence,  W.  J.  C. 

1956.  Soil  sterilization.  171  p.  George  Allen  and  Unwin  Ltd.,  London,  England. 
Lawrence,  W.  J.  C,  and  A.  C.  Hunt 

1955.  The  John  [nnes  high  rate  soil  steriliser.  John  Innes  Hort.  Inst.  Lflt.  13:  1-10. 


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Lawrence,  W.  J.  C,  and  J.  Newell 

1950.  Seed  and  potting  composts,  with  special  reference  to  soil  sterilization.  4th  ed.  166  p. 
George  Allen  and  Unwin  Ltd.,  London,  England. 

Morris,  L.  G. 

1953.  How  to  steam  sterilise  your  casing  soil.  Mushroom   Grow.  Assoc.    (Yaxley,  England) 
Bui.  45:  3  p. 

1954.  The  steam  sterilising  of  soil.  The  application  of  research  to  practice.  Brit.  Nat'l  Inst. 
Agr.  Engin.  Rept.  24:  1-22. 

Newhall,  A.  G. 

1930.    Control  of  root>knot  nematode  in  greenhouses.  Ohio  Agr.  Exp.  Sta.  Bui.  451:  1-60. 
1940.    Experiments  with  new  electric  devices  for  pasteurizing  soils.  New  York  (Cornell)   Agr. 
Exp.  Sta.  Bui.  731:  1-38. 

1953.  How  large  a  generator  is  needed  to  steam  your  soil.  New  York  State  Flower  Growers 
Bui.  No.  91:  12. 

Newhall,  A.  G.,  C.  Chupp,  and  C.  E.  F.  Guterman 

1940.    Soil  treatments  for  the  control  of  diseases  in  the  greenhouse  and  the  seedbed.  New 
York  (Cornell)  Agr.  Exp.  Sta.  Bui.  217  (rev.)  :  1-58. 
Newhall,  A.  G.,  and  W.  T.  Schroeder 

1951.  New  flash-flame  soil  pasteurizer.  New  York  (Cornell)  Agr.  Exp.  Sta.  Bui.  875:  1-19. 
Peterson,  M.  L. 

1942.    An  inexpensive  soil  sterilizer.  Amer.  Soc.  Agron.  Jour.  34:  674-76. 
Roistacher,  C.  N.,  and  K.  F.  Baker 

1956.    An  inexpensive  multipurpose  soil  steamer.  Phytopathology  46:  329-33. 
Roll-Hansen,  J. 

1949.    Damping  av  jord.  68  p.  A/L  Norsk  Gartnerforenings  Forlag,  Oslo,  Norway. 
Rudd,  W.  N. 

1893.    Killing  grubs  in  soil.  Amer.  Florist  (Chicago,  111.)  9  (278)  :  171. 
Schmitz,  S.  S. 

1954.  Steam  sterilizing  at  lower  cost.  Florists'  Rev.  114  (2961)  :  19,  45. 
Seeley,  J.  G. 

1954.  Steam  sterilization  of  greenhouse  soils.  Pennsylvania  Flower  Growers  Bui.  No.  39  :  1-6. 
Senner,  A.  H. 

1934.  Application  of  steam*  in  the  sterilization  of  soils.  U.  S.  Dept.  Agr.  Tech.  Bui.  443:  1-20. 
Tarrant  Mfg.  Co. 

1955.  Roto-therm  soil  pasteurizer.  2  p.  Tarrant  Mfg.  Co.,  Saratoga  Springs,  N.Y 
Tavernetti,  J.  R. 

1935.  Characteristics  of  the  resistance  type  soil  sterilizer.  Agr.  Engineering  (St.  Joseph,  Mich.) 
16:  271-74. 

1942.    A  continuous  soil  pasteurizer.  Agr.  Engineering  (St.  Joseph,  Mich.)  23:  255-56,  261. 
Thomas,  C.  A. 

1954.  Some  greenhouse  soil  pests  and  their  control — nematodes.  Pennsylvania  Flower  Growers 
Bui.  No.  47:  1-5,8. 

Webrer,  R. 

1956.  Mr.  Jackson's  soil  steamer  keeps  on  moving.  The  Grower  (London)  45:  208-9. 

Section  1 1 

Hamner,  O.  H.,  and  F.  C.  Amstutz 

1955.  Apparatus  for  more  rapid  vaporization  of  methyl  bromide.  Down  to  Earth    (Midland, 
Mich.)  11  (2):  11-13. 

Jefferson,  R.  N.,  and  A.  E.  Pritchard 

1956.  Pest   control  guide   for   California   floricultural   crops.   California   Agr.  Ext.   Serv.   Lflt. 
66:  1-11. 

Kendrick,  J.  B.,  Jr.,  and  G.  A.  Zentmyer 

1957.  Recent  advances  in  control  of  soil  fungi.  In:  Advances  in  pest  control  1:  219-75.  Inter- 
science  Publishers,  Inc.,  New  York,  N.Y. 

Lear,  B. 

1951.    Use  of   methyl   bromide   and  other  volatile   chemicals   for   soil   fumigation.   New   York 
(Cornell)  Agr.  Exp.  Sta.  Mem.  303:  1^8. 


[293] 


Lear,  B.,  and  W.  F.  Mai 

1952.  Methyl  bromide  for  disinfesting  burlap  bags  and  machinery  to  help  prevent  spread  of 
golden  nematode  of  potatoes.  Phytopathology  42:  489-92. 

Martin,  W.  J.,  N.  L.  Horn,  and  J.  A.  Cox 

1955.    Fumigation  of  bell  pepper  seed  beds  for  controlling  damping-off  caused  by  Rhizoctonia 
solani.  Plant  Dis.  Reptr.  39:  678-81. 
Munnecke,  D.  E.,  and  J.  Ferguson 

1953.  Methyl  bromide  for  nursery  soil  fumigation.  Phytopathology  43:  375-77. 
Munnecke,  D.  E.,  and  D.  L.  Lindgren 

1954.  Chemical  measurements  of  methyl  bromide  concentration   in   relation  to  kill  of  fungi 
and  nematodes  in  nursery  soil.  Phytopathology  44:  605-6. 

Newhall,  A.  G.,  and  B.  Lear 

1948.  Soil  fumigation  for  nematode  and  disease  control.  New  York   (Cornell)   Agr.  Exp.  Sta. 
Bui.  850:  1-32. 

Pritchard,  A.  E. 

1949.  California  greenhouse  pests  and  their  control.  California  Agr.  Exp.  Sta.  Bui.  713:  1-72. 
Sciaroni,  R.  H. 

1955.  Terraclor  for  Rhizoctonia  disease  of  carnation.  California  Agr.  Ext.  Serv.,  Alameda  and 
San  Mateo  Counties,  Flower  Notes  No.  18  :  5. 

Sciaroni,  R.  H.,  and  R.  D.  Raabe 

1955.  Rhizoctonia  disease  control  in  carnations  with  PCNB.  California  Agr.  Ext.  Serv.,  Alameda 
and  San  Mateo  Counties,  Flower  Notes  No.  16  :  6. 

Stark,  F.  L.,  Jr. 

1948.    Investigations  of  chloropicrin  as  a  soil  fumigant.  New  York   (Cornell)    Agr.  Exp.  Sta. 
Mem.  278:  1-61. 

Section  12 

Baker,  K.  F.,  and  F.  D.  Heald 

1934.  Investigations  on  methods  of  control  of  the  blue-mold  decay  of  apples.  Washington  Agr. 
Exp.  Sta.  Bui.  304:  1-32. 

Huber,  G.  A. 

1935.  The  use  of  sodium  hypochlorite  solutions  as  disinfecting  agents.  Better  Fruit  (Portland, 
Ore.)  29  (12)  :  5-6. 

Klotz,  L.  J.,  and  T.  A.  DeWolfe 

1952.    Steam  sterilization  of  citrus  field  and  storage  boxes.  Citrus  Leaves  32  (12)  :  20,  35. 
Lear,  B.,  and  W.  F.  Mai 

1952.    Methyl  bromide  for  disinfesting  burlap  bags  and  machinery  to  help  prevent  spread  of 
golden  nematode  of  potatoes.  Phytopathology  42:  489-92. 
Roistacher,  C.  N. 

1952a.  Methods  used  in  sterilizing  nursery  flats.  Pacific  Coast  Nurseryman  11  (5)  :  17,  34-36. 

19526.  Phytotoxicity  of  formaldehyde  residue  on  flats.  Phytopathology  42:  171-72. 
Roistacher,  C.  N.,  and  K.  F.  Baker 

1954.    Disinfesting  action  of  wood  preservatives  on  plant  containers.  Phytopathology  44:  65-69. 
Wellman,  R.  H.,  and  F.  D.  Heald 

1938.    Steam   sterilization   of   apple   boxes   for   blue   mold.   Washington   Agr.   Exp.   Sta.   Bui. 
357:  1-16. 

Section  1  3 

Baker,  K.  F. 

1948.    The   significance   of   disease-free   seed   and   propagating   material.   Florists'   Exch.    110 

(9)  :  21,  30,  58-59. 
1952.    A  problem  of  seedsmen  and  flower  growers — seed-borne  parasites.  Seed  World  70  (11)  : 
38,  40,  44,  46-47. 

1956.  Development  and  production  of  pathogen-free  seed  of  three  ornamental  plants.  Plant  Dis. 
Reptr.  Suppl.  238:  68-71. 

BAKER,  K.  F.,  and  P.  A.  CHANDLER 

1956.    Development   and  product  inn  of  pathogen-free  propagative  material  of  foliage  and  suc- 
culenl  plants.  Plant  Dis.  Reptr.  Suppl.  238:  88  90 


|  2<ri  | 


Baker,  K.  F.,  and  K.  Cummings 

1943.    Control  of  Pythium  root  rot  of  Aloe  variegata  by  hot-water  treatment.  Phytopathology 
33:  736-38. 
Baker,  K.  F.,  and  L.  H.  Davis 

1950.    Some  diseases  of  ornamental  plants  in  California  caused  by  species  of  Alternaria  and 
Stemphylium.  Plant  Dis.  Reptr.  34:  403-13. 
Baker,  K.  F.,  and  W.  C.  Snyder 

1950.    Plant  diseases.  Restrictive  effect  of  California  climate  on  some  vegetable,  flower,  grain 
diseases.  California  Agr.  4  (8)  :  3,  15-16. 
Brierley,  P.,  and  C.  J.  Olson 

1956.    Development  and  production  of  virus-free  chrysanthemum  propagative  material.  Plant 
Dis.  Reptr.  Suppl.  238:  63-67. 
Chandler,  P.  A. 

1953.    How  to   avoid  diseases  in  plants  and   containers.   Pacific  Coast  Nurseryman    12    (6)   : 
15-16,  44-45. 
Chitwood,  B.  G. 

1956.    Root  nematode  problems  in  roses.  Plant  News  (Blue  Point,  N.Y.)  6  (2)  :  1. 
Courtney,  W.  D.,  E.  P.  Breakey,  and  L.  L.  Stitt 

1947.    Hot-water  tanks  for  treating  bulbs  and  other  plant  materials.  Washington   Agr.  Exp. 
Sta.  Pop.  Bui.  184:  1-16. 
Davis,  L.  H. 

1953.  Black  cane  rot  of  Syngonium  auritum.  (Abs.)  Phytopathology  43:  586. 
Dimock,  A.  W. 

1943.  Coming    up-to-date    on    Verticillium    wilt    and    Septoria    leafspot    of    chrysanthemums. 
Chrysanth.  Soc.  Amer.  Bui.  11  (1)  :  3-10. 

1956.    Production  of  chrysanthemum  propagating  material   free   of  certain   major  pathogens. 
Plant  Dis.  Reptr.  Suppl.  238:  59-62. 
Dimock,  A.  W.,  and  K.  F.  Baker 

1944.  Hot-water  treatment  for  control  of  Phytophthora  root  rot  of  calla.  Phytopathology  34: 
979-81. 

Durbin,  R.  D. 

1956.    Hot-water  seed  treatment  for  control  of  a  root  rot  complex  of  Strelitzia  reginae.  Plant 
Dis.  Reptr.  40:  1116. 
Gasiorkiewicz,  E.  C,  and  C.  J.  Olson 

1956.    Progress  in  the  development  and  production  of  virus-free  carnation  varieties.  Plant  Dis. 
Reptr.  Suppl.  238:  77-80. 
Hansen,  H.  N.,  and  W.  C.  Snyder 

1947.    Gaseous  sterilization  of  biological  materials  for  use  as  culture  media.  Phytopathology 
37:  369-71. 
Holmes,  F.  O. 

1955.  Elimination  of  spotted  wilt  from  dahlias  by  propagation  of  tip  cuttings.  Phytopathology 
45:  224-26. 

1956a.  Elimination  of  foliage  spotting  from  sweetpotato.  Phytopathology  46:  502-4. 
19566.  Elimination  of  aspermy  virus  from  the  Nightingale  chrysanthemum.   Phytopathology 
46:  599-600. 
Hughes,  C.  G. 

1954.  The   eradication   of   sugar-cane   diseases   in   Queensland.   Fifth    Commonwealth   Mycol. 
Conf.  Rept.,  p.  59-64.  Commonwealth  Mycol.  Inst.,  Kew,  England. 

Kendrick,  J.  B.,  Sr.,  and  K.  F.  Baker 

1942.    Bacterial  blight  of  garden  stocks  and  its  control  by  hot-water  seed  treatment.  California 
Agr.  Exp.  Sta.  Bui.  665:  1-23. 
Martin,  C. 

1954.    Recherches  sur  les  maladies  a  virus  du  dahlia.  Ann.  Epiphyties  5:  63-78. 
Miller,  P.  W. 

1954.    Inactivation  of  non-persistent  viruses  in  strawberry  plants  by  hot-air  treatment.  Plant 
Dis.  Reptr.  38:  827-31. 

MUNNECKE,  D.  E. 

1956.  Development  and  production  of  pathogen-free  geranium  propagative  material.  Plant  Dis. 
Reptr.  Suppl.  238:  93-95. 


[295] 


Norms,  D.  0. 

1954.    Development  of  a  virus-free  stock  of  Green  Mountain  potato  by  treatment  with  malachite 
green.  Austral.  Jour.  Agr.  Res.  5:  658-63. 
Quak,  F. 

1957.    Meristeemcultuur,    gecombineerd    met    warmtebehandelung,    voor    het    verkrijgen    van 
virusvrije  anjerplanten.  Tijdschr.  Plantenziekt.  63:  13-14. 
Snyder,  W.  C,  and  H.  N.  Hansen 

1947.  Advantages  of  natural  media  and  environments  in  the  culture  of  fungi.  Phytopathology 
37:  420-21. 

Tammen,  J.,  R.  R.  Baker,  and  W.  D.  Holley 

1956.    Control  of  carnation  diseases  through  the  cultured-cutting  technique.  Plant  Dis.  Reptr. 
Suppl.  238:  72-76. 
Vaughan,  E.  K. 

1956.    A  method  for  eliminating  the  red-stele  fungus  from  valuable  strawberry  stocks.  Phy- 
topathology 46:  235-36. 
Watson,  R.  D.,  L.  Coltrin,  and  R.  Rorinson 

1951.  The  evaluation  of  materials  for  heat  treatment  of  peas  and  beans.  Plant  Dis.  Reptr.  35: 
542^4. 

West,  E. 

1942,  1943.  Host  relations  and  factors  influencing  the  growth  and  parasitism  of  Sclerotium  rolfsii 
Sacc.  Florida  Agr.  Exp.  Sta.  Ann.  Rept.  1942:  93-94;  1943:  83-84. 
Wilhelm,  S.,  and  R.  D.  Raare 

1956.    Culture-indexing  of  budwood  to  provide  Verticillium-free  greenhouse  roses.  Plant  Dis. 
Reptr.  Suppl.  238:  85-87. 
Young,  R.  A. 

1954.    Dissemination  of  plant  pathogens  on  nursery  and  ornamental  plants.  Plant  Dis.  Reptr. 
38:  417-20. 

Section  14 

Ark,  P.  A. 

1937.    Little-leaf  or  rosette  of  fruit  trees.  VII.  Soil  microflora  and  little-leaf  or  rosette  disease. 
Amer.  Soc.  Hort.  Sci.  Proc.  34:  216-21. 
Ferguson,  J. 

1953.    The  effect  of  soil  treatment  on  beneficial  soil  organisms.  Pacific  Coast  Nurseryman  12 
(7)  :  15,  48. 
Gerretsen,  F.  C. 

1937.    Manganese  deficiency  of  oats  and  its  relation  to  soil  bacteria.  Ann.  Bot.  n.  s.  1:  207-38. 

1948.  The  influence  of  microorganisms  on  the  phosphate  intake  by  the  plant.  Plant  and  Soil 
1:  51-81. 

Katznelson,  H.,  A.  G.  Lochhead,  and  M.  I.  Timonin 

1948.    Soil  microorganisms  and  the  rhizosphere.  Bot.  Rev.  14:  543-87. 
Martin,  J.  P.,  and  J.  O.  Ervin 

1952.  Soil  organisms — fact  and  fiction.  Western  Grower  and  Shipper  23    (6)  :  30-32,  34-36, 
38-39. 

Meiklejohn,  J. 

1953.  The  nitrifying  bacteria:  a  review.  Jour.  Soil  Sci.  4:  59-68. 
Nightingale,  G.  T. 

1937,  1948.  The  nitrogen  nutrition  of  green  plants.  Bot.  Rev.  3:  85-174;  14:  185-221. 
Waksman,  S.  A. 

1932.    Principles  of  soil  microbiology.  2d  ed.  894  p.  Williams  and  Wilkins  Co.,  Baltimore,  Md. 

1952.    Soil  microbiology.  356  p.  John  Wiley  and  Sons,  Inc.,  New  York,  N.Y 
Waksman,  S.  A.,  and  R.  L.  Starkey 

1931.    The  soil  and  the  microbe.  260  p.  John  Wiley  and  Sons,  Inc.,  New  York,  N.Y. 

Section  15 
Ark,  P.  A. 

1937.    Variability  in  the  fire-blight  organism,  Erwinia  amylovora.  Phytopathology  27:  1-28. 
Baker,  K.  F. 

1917.    Seed  transmission  of  Rhizortonia  solani  in  relation  to  control  of  seedling  damping-off. 

Phytopathology  37:  912-24. 
L953.    I'Hsariuni  wilt  of  China  aster.  In:  Plant  diseases.  U.  S.  Depl.  Agr.  Yearbk.  1953:  572-77. 

[  296  1 


Chitwood,  B.  G. 

1949.  "Root-knot  nematodes" — part  1.  A  revision  of  the  genus  Meloidogyne  Goeldi,  1837. 
Helminth.  Soc.  Wash.  Proc.  16:  90-104. 

Ferguson,  J.,  and  B.  B.  Markley 

1955.  The  masking  of  soil  treatment  effects  from  use  of  contaminated  planting  stock  (Abs.). 
Phytopathology  45:  693. 

Goodey,  T. 

1933.    Plant  parasitic  nematodes  and  the  diseases  they  cause.  306  p.  E.  P.  Dutton  and  Co., 
Inc.,  New  York,  N.Y. 
Holdeman,  Q.  L.,  and  T.  W.  Graham 

1954.    Effect  of  the  sting  nematode  on  expression  of  Fusarium  wilt  in  cotton.  Phytopathology 
44:  683-85. 
Kendrick,  J.  B.,  Jr. 

1951.  The  influence  of  temperature  upon  the  incidence  of  Rhizoctonia  root  rot  of  lima  beans. 
(Abs.)  Phytopathology  41:  20. 

Lacey,  M.  S. 

1939.  Studies  on  a  bacterium  associated  with  leafy  galls,  fasciations  and  "cauliflower"  dis- 
ease of  various  plants.  Part  III.  Further  isolations,  inoculation  experiments  and 
cultural  studies.  Ann.  Appl.  Biol.  26:  262-78. 

McCubbin,  W.  A. 

1954.    The  plant  quarantine  problem.  255  p.  Ejnar  Munksgaard,  Copenhagen,  Denmark. 
Rawlings,  R.  E. 

1940.  Observations  on  the  cultural  and  pathogenic  habits  of  Thielaviopsis  basicola  (Berk, 
and  Br.)  Ferraris.  Missouri  Bot.  Gard.  Ann.  27:  561-98. 

Roth,  L.  F.,  and  A.  J.  Riker 

1943.    Life  history  and  distribution  of  Pythium  and  Rhizoctonia  in  relation  to  damping-off  of 
red  pine  seedlings.  Jour.  Agr.  Res.  67:  129-48. 
Sanford,  G.  B. 

1938.  Studies  on  Rhizoctonia  solani  Kiihn.  III.  Racial  differences  in  pathogenicity.  Canad.  Jour. 
Res.,  Sec.  C,  16:  53-64. 
Sasser,  J.  N. 

1952.  Identification  of  root-knot  nematodes  {Meloidogyne  spp.)  by  host  reactions.  Plant  Dis. 
Reptr.  36:  84-86. 

Sasser,  J.  N.,  H.  N.  Powers,  Jr.,  and  G.  B.  Lucas 

1953.  The  effect  of  root  knot  nematodes  {Meloidogyne  spp.)  on  the  expression  of  black  shank 
resistance  in  tobacco.  (Abs.)   Phytopathology  43:  483. 

Snyder,  W.  C.,  and  H.  N.  Hansen 

1940.    The  species  concept  in  Fusarium.  Amer.  Jour.  Bot.  27:  64—67. 
Stewart,  R.  N.,  and  A.  F.  Schindler 

1956.  The  effect  of  some  ectoparasitic  and  endoparasitic  nematodes  on  the  expression  of 
bacterial  wilt  in  carnations.  Phytopathology  46:  219-22. 

Stover,  R.  H. 

1950.  The  black  rootrot  disease  of  tobacco.  I.  Studies  on  the  causal  organism  Thielaviopsis 
basicola.  II.  Physiologic  specialization  of  Thielaviopsis  basicola  on  Nicotiana  tabacum. 
Canad.  Jour.  Res.  Sec.  C,  28:  445-70;  726-38. 

Tarjan,  A.  C. 

1952.    Pathogenic  behavior  of  certain  root-knot  nematodes,  Meloidogyne  spp.  on  snapdragon, 
Antirrhinum  majus  L.  Phytopathology  42:  637-41. 
Tucker,  C.  M. 

1931.    Taxonomy  of  the  genus  Phytophthora  de  Bary.  Missouri  Agr.  Exp.  Sta.  Res.  Bui.  153: 
1-208. 
Wilhelm,  S.,  and  R.  D.  Raabe 

1956.  Culture-indexing  of  budwood  to  provide  Verticillium-free  greenhouse  roses.  Plant  Dis. 
Reptr.  Suppl.  238:  85-87. 

WORMALD,  H. 

-     1945.    Physiologic  races  of  the  crown  gall  organism  in  Britain.  Brit.  Mycol.  Soc.  Trans.  28: 
134-46. 


[297] 


Section  17 

Baker,  K.  F. 

1948.    Nursery  seedlings.  Improved  methods  of  production  with  control  of  damping-off  disease. 
Pacific  Coast  Nurseryman  7  (11)  :  9,  28-29,  cover. 
Ball,  V. 

1953.  Erdprinz.  Grower  Talks   (West  Chicago,  111.)    16  (10)  :  14. 
Connelly,  J.  R. 

1943.  Technique  of  production  processes.  430  p.  McGraw-Hill  Book  Co.,  Inc.,  New  York,  N.Y. 
Harrington,  C.  C. 

1952.  Materials  handling  manual.  434  p.  Conover-Mast  Publ.,  Inc.,  New  York,  N.Y. 
Hetzel,  F.  V.,  and  R.  K.  Albright 

1941.    Belt  conveyors  and  belt  elevators.  439  p.  John  Wiley  and  Sons,  Inc.,  New  York,  N.Y. 
Hudson,  W.  G. 

1954.  Conveyors  and  related  equipment.  3d  ed.  524  p.  John  Wiley  and  Sons,  Inc.,  New  York, 
N.Y. 

Immer,  J.  R. 

1953.  Materials  handling.  591  p.  McGraw-Hill  Book  Co.,  Inc.,  New  York,  N.Y. 

KOSHKIN,  S.  J. 

1932.    Modern  materials  handling.  488  p.  John  Wiley  and  Sons,  Inc.,  New  York,  N.Y. 

MUTHER,  R. 

1944.  Production-line  technique.  320  p.  McGraw-Hill  Book  Co.,  Inc.,  New  York,  N.Y. 
Staniar,  W. 

1950.  Plant  engineering  handbook.  2,007  p.  McGraw-Hill  Book  Co.,  Inc.,  New  York,  N.Y. 
Stocker,  H.  E. 

1951.  Materials  handling.  Principles,  equipment,  and  methods.  2d  ed.  330  p.  Prentice-Hall, 
Inc.,  New  York,  N.Y. 


GLOSSARY  OF  TERMS,  AND  SOME  COMPUTATION  METHODS 

Aerobic — Living  or  active  only  in  the  presence  of  oxygen. 

Anaerobic — Living  or  active  in  the  absence  of  free  oxygen. 

Antibiotic — A  chemical  substance  produced  by  an  organism  which  inhibits  the  de- 
velopment of  a  microorganism. 

Bacterium — A  microscopic  unicellular  plant  that  lacks  chlorophyll  and  reproduces 
by  dividing  into  two  parts;  some  cells  may  develop  into  resistant  spores. 

Boiler  horsepower — Unit  of  boiler  capacity;  different  from  mechanical  horse- 
power. Is  equal  to  heat  required  to  convert  34.5  pounds  of  water  at  212°  F  into 
steam.  Since  it  requires  970.3  B.t.u.  per  pound  to  convert  water  to  steam,  1 
horsepower  equals  33,475  B.t.u.  The  term  is  indefinite  as  commercially  used 
because  it  is  determined  by  measuring  the  cross-sectional  area  of  actual  heat 
exchange  between  the  source  of  heat  and  the  water,  without  reference  to  ef- 
ficiency or  design.  Boiler  capacity  is  best  expressed  as  pounds  of  steam  gen- 
erated per  hour.  For  calculated  conversion  to  boiler  horsepower  see  table  14. 

Boiler  output  of  steam — Roughly  calculated  for  small  boilers  by  injecting  the 
steam  into  a  weighed  container  of  cold  water  for  a  measured  time  (water  must 
not  be  heated  to  point  of  vaporizing),  and  the  container  then  reweighed.  The 
difference  represents  the  pounds  of  steam  delivered  to  the  container  during  the 
given  number  of  minutes  and  may  easily  be  converted  to  pounds  per  hour. 

B.t.u.  (British  thermal  unit) — Unit  for  quantity  of  heat;  amount  of  heat  required 
to  raise  the  temperature  of  1  pound  of  water  1  degree  F. 

B.t.u.  requirement  for  heating  soil — See  Section  8,  "Cost  of  Steaming  Soil, 
Heal  Requirements." 

[298] 


Buffering  capacity — The  relative  ability  of  a  soil  solution  to  resist  change;  as 
used  in  this  manual  it  refers  to  pH.  It  is  also  used  in  a  biological  sense  to  in- 
dicate the  ability  of  a  population  of  soil  microorganisms  to  resist  change. 

Conductance — The  reciprocal  of  the  resistance  (in  ohms)  of  an  electrolyte  to  the 
passage  of  electrical  current.  The  greater  the  salt  content  of  the  water,  the 
greater  the  conductance.  Measured  by  a  Wheatstone  bridge  (Solubridge)  and 
expressed  as  millimhos  per  cm  (EC  x  103)  or  mhos  per  cm  x  100,000  (EC  x 
105). 

Efficiency  of  boiler — May  be  approximated  (in  per  cent)  by  multiplying  the  heat- 
ing value  of  the  fuel  in  B.t.u.  (Sec.  10)  by  the  quantity  used  per  hour,  and 
dividing  this  figure  into  the  product  of  the  output  of  steam  in  pounds  per  hour 
(see  sees.  9  and  10,  and  "Boiler  output  of  steam,"  above)  and  1,150  B.t.u. 
(fig.  77).  Multiply  the  result  by  100  to  convert  to  percentage. 

Efficiency  of  soil  steaming  operation — Multiply  970  B.t.u.  plus  half  the  dif- 
ference between  212°  F  and  the  initial  soil  temperature  (fig.  77)  by  the  pounds 
of  steam  necessary  to  heat  the  volume  of  soil  to  212°  (see  Sec.  9  and  "Steam 
delivered  to  treatment  site,"  below) .  Divide  this  figure  by  the  product  of  the 
B.t.u.  required  to  heat  1  cubic  foot  of  soil  (see  Sec.  8,  "Heat  Requirements") 
and  the  cubic  feet  of  soil  treated.  This  will  provide  the  approximate  efficiency. 
The  calculation  may  be  stated  as  a  mathematical  formula  thus: 

„_„„  212°  F- initial  soil  temperature "1     „  ,      ,  nnn. 

970  B.t.u.  -f  x  lbs.  steam  required  to  heat  to  212 


B.t.u.  required  per  cu.  ft.  x  cu.  ft.  of  soil  treated 
Multiply  the  result  by  100  to  convert  to  percentage. 

Efficiency  of  steam  distribution  system — The  pounds  of  steam  per  hour  at  the 
boiler,  minus  the  pounds  of  steam  per  hour  at  the  treatment  site,  gives  the 
pounds  per  hour  lost  in  distribution.  This  figure,  divided  by  the  pounds  of 
steam  at  the  boiler  and  multiplied  by  100,  gives  the  percentage  loss  in  distribu- 
tion. 

Fungistatic — Preventing  growth  of  a  fungus  without  killing  it.  Fungicidal  is  killing 
the  fungus. 

Fungus — A  primitive  plant  that  lacks  chlorophyll  and  is  undifferentiated  into  root, 
stem,  or  leaves;  it  reproduces  by  spores. 

Heat  capacity — See  "Specific  heat,"  below. 

Heat  conductivity — Rate  of  heat  transfer  by  conduction  through  a  material;  usu- 
ally expressed  as  calories  per  second  per  sq.  cm  for  1  cm  thickness  and  1  degree  C 
temperature  differential.  Comparative  heat  conductivities  of  some  materials  are : 
water  0.0014,  peat  soil  0.00015,  cork  0.0001,  air  0.00006;  most  dry  soils  are  of 
the  order  0.0007,  increasing  with  moisture  content  up  to  about  that  of  water. 

Heat  requirements — Dry  soil  requires  about  1.200  B.t.u.  per  cubic  foot  per  100 
degrees  F  of  temperature  rise.  Soil  moisture  requires  about  60  B.t.u.  per  1  per 
cent  of  water  by  weight  for  each  100  degrees  F  of  temperature  rise. 

Host — A  plant  that  is  or  may  be  attacked  by  a  parasite  or  virus. 

Infect,  infection — The  process  by  which  a  parasite  gains  entrance  and  becomes 
established  in  the  host. 

Infest — To  contaminate  with  microorganisms  without  infection  or  parasitism.  Thus 
a  host  may  be  infested  or  infected,  whereas  a  nonliving  object  can  only  be  in- 
fested. 

[299] 


Inoculum  potential — The  relative  quantity  of  a  parasite  in  the  environment  of 
the  plant;  interaction  of  inoculum  potential,  environment,  virulence  of  the 
strain,  and  host  susceptibility  determine  the  incidence  of  a  disease. 

Kilowatt-hour  (K.w.h.) — Unit  of  electrical  power  or  energy  consumption; 
amount  of  energy  developed  by  1  kilowatt  per  hour,  equal  to  3,411  B.t.u.  or 
roughly  1/10  boiler  horsepower.  For  calculated  conversion  to  boiler  horse- 
power see  table  14. 

Mycelium — The  threadlike  vegetative  structure  of  a  fungus. 

Nematode — Minute  wormlike  animal;  some  types  feed  on  or  in  plants. 

Osmotic  pressure — The  negative  pressure  that  influences  the  rate  of  diffusion  of 
water  through  a  semipermeable  membrane  such  as  a  cell  wall. 

Parasite — An  organism  that  lives  in  or  on,  and  obtains  its  nutrients  from,  a  host 
plant. 

Pathogen — An  organism  or  virus  capable  of  causing  disease  in  a  host  plant. 

Pound  of  steam — Quantity  of  steam  produced  by  evaporating  1  pound  of  water; 
970  B.t.u.  per  pound  are  required  to  evaporate  it,  and  the  same  number 
are  available  to  heat  soil  to  212°  F  as  it  condenses. 

Retardant  or  antagonistic  microorganisms — Microorganisms  which  delay 
growth  of,  or  disease  production  by,  other  microorganisms.  This  effect  may  be 
brought  about  by  (1)  competing  with  other  microorganisms  for  conditions 
essential  for  growth,  (2)  parasitizing  them,  or  (3)  exerting  an  antibiotic  effect 
on  them. 

Rogue — To  remove  diseased  plants  from  a  crop,  usually  to  obtain  disease-free  stock. 

Saline  soil — A  soil  containing  soluble  salts  in  such  quantities  that  they  interfere 
with  the  growth  of  a  crop  plant.  This  is  distinguished  from  alkali  soils,  which 
contain  sufficient  exchangeable  sodium  to  interfere  with  the  growth  of  a  crop 
plant.  A  soil  may  be  saline,  nonsaline-alkali,  or  saline-alkali.  Alkali  soil  is 
distinct  from  alkaline  soil,  which  has  a  pH  reaction  above  neutrality  and  may 
or  may  not  be  an  alkali  soil. 

Salinity  measurement — Conductance  is  measured  by  California  nurserymen  with 
Solubridge  equipment.  Either  the  RD-26  or  the  RD-15  may  be  used,  the  latter 
giving  scale  readings  100  times  the  former.  This  can  be  converted  to  approxi- 
mate ppm  by  multiplying  the  RD-26  reading  by  650  and  the  RD-15  by  6.5. 

Saprophyte — An  organism  that  obtains  its  nutrients  from  nonliving  organic 
matter. 

Saturated  steam — Steam  still  in  contact  with  the  water  from  which  it  was  formed. 
Almost  all  commercial  steam  used  for  soil  treatment  is  of  this  type,  and  still 
contains  water  droplets.  It  is  of  two  kinds:  (1)  Low  pressure,  "pressureless," 
or  free-flowing  steam  expands  freely  through  the  lines  until  it  condenses.  Its 
pressure  usually  is  below  5  pounds  per  square  inch  and  its  temperature  only 
slightly  above  212°  F.  (2)  Steam  that  is  not  permitted  to  expand  freely  and  de- 
velops pressure;  the  temperature  also  rises. 

Sclerotium — A  compact,  resistant,  dormant  mass  of  mycelium  by  which  the  fungus 
survives  unfavorable  conditions. 

Soil  moisture — Usually  determined  by  weighing  a  sample  both  before  and  after 
water  is  driven  off  at  221  °  or  230°  F.  The  weight  of  water  lost,  divided  by  the 
dry  weight,  gives  the  moisture  percentage  by  weight. 


|  300  ] 


Specific  heat — Ratio  of  the  heat  capacity  of  a  substance  to  that  of  water.  The  heat 
capacity  is  the  number  of  calories  of  heat  necessary  to  raise  the  temperature  of 
1  gram  of  a  substance  1  degree  C.  For  purposes  of  comparison,  the  specific  heat 
of  water  is  about  1.0,  those  of  steam  and  wood  about  0.5,  those  of  dry  air,  dry 
soil,  and  aluminum  about  0.2,  and  that  of  steel  about  0.1.  Thus,  it  takes  about 
twice  as  many  B.t.u.  to  heat  1  pound  of  water  as  1  pound  of  steam,  and  about  5 
times  as  many  as  for  dry  soil  or  air. 

Spore — A  reproductive  body  of  fungi  and  bacteria  by  which  they  spread  or  survive 
unfavorable  conditions. 

Steam  delivered  to  treatment  site — The  same  procedure  as  for  "Boiler  output 
of  steam,"  above,  is  carried  out  at  the  point  of  steam  entry  into  the  container. 
The  pounds  per  hour  delivered  at  this  point  may  then  be  computed. 

Steam  pressure — The  pressure  exerted  per  square  inch  of  surface  by  the  steam. 
Water  expands  1,600  times  as  it  is  converted  to  steam,  building  up  pressure 
in  the  confined  area  if  not  permitted  to  escape  freely.  This  is  measured  in  gauge 
pressure  per  square  inch.  This  is  not  related  to  the  quantity  (pounds  per  hour) 
of  steam,  and  only  slightly  to  its  efficiency  in  steaming  soil.  The  temperature  in- 
creases with  the  pressure  (fig.  77) . 

Substrate — Material  in  or  on  which  an  organism  grows  or  to  which  it  is  attached. 

Superheated  steam — Steam  removed  from  its  water  source  and  heated,  much  as 
air  is  warmed.  Because  water  in  the  saturated  steam  evaporates  on  superheating, 
this  type  of  steam  is  dry.  It  transmits  about  47  B.t.u.  per  100  degrees  F  of  super- 
heat more  than  saturated  steam. 

Water  mold — A  primitive  type  of  fungus  (one  of  the  Phycomycetes)  that  lives  in 
very  moist  soil;  some  are  able  to  parasitize  plants. 


WEIGHTS  AND  MEASURES 

1  bu.  =  1.24 cu.  ft.;  0.05  cu.  yd.;  3  standard  southern  California  flats. 

1  cc  (cubic  centimeter)  =  0.061  cu.  in.;  0.034  fl.  oz. ;  0.001  liter. 

1  cu.  ft.  =  0.037  cu.  yd. ;  0.804  bu. ;  28,317  cc ;  1,728  cu.  in. 

1  cu.  yd.  =  27  cu.  ft.;  46,656  cu.  in;  21.70  bu.;  fills  60  standard  southern  California 
flats  (18  x  18  x  3  in.)  with  soil. 

1  gal.  of  water  =  231  cu.  in. ;  0.13  cu.  ft. ;  8.34  lb. ;  8  pints. 

1  lb.  of  water  (60°  F)  =  0.12  gal. ;  0.016  cu.  ft. ;  27.71  cu.  in. 

1  lb.  per  cu.  yd.  -  1  lb.  per  27  cu.  ft.;  1  lb.  per  21.7  bu.;  0.74  oz.  per  bu. ;  0.59  oz.  per 
cu.  ft.;  3.6  grams  per  gal. 

1  lb.  =  453.6  grams;  16  oz. 

1  ml  (milliliter)  =  1  cc  (see  above) . 

1  oz.  (avdp.)  =  28.3  grams;  0.0625  lb. 

1  oz.  (fluid)  =  29.6  cc  or  ml;  0.0625  pint;  2  tablespoons. 

1  pint  =  16  fl.  oz.;  473.2  cc  or  ml;  0.125  gal.;  32  tablespoons. 


[301] 


SOURCES  OF  EQUIPMENT  AND  MATERIALS2 

Boilers,  package  and  flash  type 

Cleaver-Brooks  Self-contained  Boilers.  Cleaver-Brooks  Co.,  Milwaukee,  Wis. 

Cyclotherm  Generator.  Cyclotherm  Division,  U.  S.  Radiator  Corp.,  Oswego  1,  N.Y. 

Francis  Steam  Generators  (1%,  2%,  3%,  5,  8,  10,  15,  25,  40,  60  h.p.)  Francis 
Steam  Generator  Co.,  Inc.,  Bellflower,  Calif. 

Hypressure  Steam  Jenny.  Metropolitan  Greenhouse  Mfg.  Co.,  Brooklyn  37,  N.Y. 

Iron  Horse  Electric  Steam  Generator.  North  Bay  Electric  Works  Inc.,  San  Rafael, 
Calif. 

Sellers  Immersion  Steam  Boilers.  Sellers  Engineering  Co.,  Chicago  40,  111. 

Steam-flo  soil  sterilizer.  Rough  Bros.,  Cincinnati  23,  Ohio. 

Steam  Master  steam  generator.  Lord  and  Burnham,  Mt.  Eden,  Calif. 
Boilers,  regular,  low  or  high  pressure 

There  are  many  types,  sizes,  and  makes  available,  both  new  and  secondhand,  that 
can  be  used  for  soil  steaming.  For  information  on  these  consult  a  heating  engi- 
neer. 
Fertilizing  equipment 

Proportioner.  Smith  Precision  Fertilizer  Injector.  Smith  Precision  Products  Co., 
1135  Mission  St.,  South  Pasadena,  Calif. 
Hot-water-treating  equipment 

Tanks.  National  Blower  and  Sheet  Metal  Co.,  1129  St.  Paul  Ave.,  Tacoma  2,  Wash. 

Thermometer,  chemical.  0°  to  220°  F,  12  in. -length,  full  immersion,  nitrogen-filled. 
Available:  1,  2. 

Thermometer,  precision  grade,  normal,  30°  to  220°  F  in  ^5°  F  subdivisions.  For 
calibrating  and  standardizing  the  above  thermometers.  Available:  1,  2. 

Thermometer  with  expanded  scale  in  critical  range.  Custom-made,  with  expanded 
ranges  available  at  86°  to  113°,  113°  to  140°,  or  104°  to  131°  F.    N.  V.  Pieter- 
man,  Division  of  M.  van  Baaren,  Den  Haag,  Netherlands. 
Planting  equipment 

Erdprinz  Planter.  Imported  by  The  Plant  Products  Corp.,  Blue  Point,  Long  Island; 
under  test  by  Geo.  J.  Ball,  Inc.,  West  Chicago,  111. 

Vacuum  Plate  Seed  Planters.  Ames  Powercount  Co.,  Brookings,  South  Dakota. 

Planting  machines  for  cans  and  pots.  Oki  Nursery,  Perkins,  Calif. 
Soil-fumigation  equipment 

Aeron  Cover  (vinyl-coated  nylon) .  Expensive,  but  very  durable.  Available:  3,  4,  6. 

Hand  and  machine  injectors,  etc.  Available:  3,  4,  6. 

Ultron  Cover   (polyvinyl  chloride).  Expensive,  but  very  durable.  Available:  3, 
4,6. 

Visqueen  Cover  (polyethylene).  Cheapest,  least  durable  covers;  excellent  for  pur- 
pose. Available:  3,  4,  6. 

'  Dealers  supplying  several  items  are  listed  numerically  under  Equipment  Dealers  at  the  end 
of  this  section;  numbers  refer  to  these  sources.  Types  of  soil-steaming  equipment  refer  to  those 
in  Section  10.  Materials  and  equipment  mentioned  by  trade  names  in  the  text  are  listed,  with 
some  sources  where  they  were  available  when  this  manual  was  prepared.  No  endorsement  of 
these  materials  or  sources  is  intended,  nor  is  criticism  implied  of  similar  products  or  sources  not 
mentioned. 


|  302  ] 


Soil-steaming  equipment 

Aluminum  pipe.  Armco  Drainage  and  Metal  Products  Co.,  Berkeley  and  Los  An- 
geles. Also  see  dealers  in  portable  irrigation  pipe. 
Duratex  Cover.  American  Associated  Co.,  Red  Bank,  N.J. 
Elliott  Steam  Soil  Sterilizer.   (Revolving  screw,  with  steam  generator;  type  29). 

Elliott  Manufacturing  Co.,  Fresno,  Calif. 
Fiberthin  Cover  (U.  S.  Rubber  Co.).  Colorado  Tent  and  Awning  Co.,  Denver  2. 

Colo. 
Hardy  and  Dillon  Electric  Heater  Assembly.  (Immersion  heaters,  type  14) .  Hardy 

and  Dillon  Co.,  Inc.,  Boston  8,  Mass. 
Pike's  Peak  Plastic  Covering.  Available:  5. 
Porous  canvas  hose  for  Thomas  Method.  Available:  5. 
Porto  Portable  Soil  Sterilizer.  (Mobile  bin;  type  2).  Fairview  Plant  Farm,  Janes- 

ville,  Wis. 
Roto-therm  Soil  Pasteurizer.    (Horizontal  rotating  drum,  external  flame,  batch, 

type  35) .  Tarrant  Mfg.  Co.,  Saratoga  Springs,  N.Y. 
Sisalkraft  Paper  Cover.  Often  available  at  lumberyards. 
Snyder's  Plastic  Covering.  M.  L.  Snyder  and  Son,  Cincinnati  27,  Ohio. 
Steam  box  (types  4a,  4b).  Fabricated  on  order  by  H.  Ernest  Ashton  Plumbing 

and  Heating,  1707  S.  Hoover  St.,  Los  Angeles  6,  Calif. 
Stericover  (Goodyear  Rubber  Co.).  Jednak  Floral  Co.,  Columbus  16,  Ohio. 
Steriltex  and  Sterilite.  Dahm's  Greenhouse  and  Garden  Supplies,  Des  Plaines,  111. 
Tarco  Flash-flame  Soil  Sterilizer.  (Rotating  drum,  with  internal  blow  torch;  type 

30).  Tarrant  Mfg.  Co.,  Saratoga  Springs,  N.Y. 
Tempil  Pellets.  Available:  1,  2. 

Tufedge  Cover.  Cleveland  Cotton  Products,  Cleveland  14,  Ohio. 
Velon  Fumicover  (Firestone  Rubber  Co.).  Industrial  Fumigation  Co..  Chicago. 

111. 
Visqueen.  See  "Soil-fumigation  equipment,"  above. 
Soil-testing  equipment 

Balance,  Harvard  trip,  triple  beam,  2,000-gram  capacity.  Available:  1,  2. 
Biichner  funnel  filter,  porcelain,  71-  or  75-mm  inside  diameter.  Available:  1,  2. 
Centimeter  rule.  Available:  1,  2. 
Filter  flask,  Erlenmeyer  Pyrex  heavy  wall,  with  tubulation,  500  ml.  Available: 

1,2. 
Filter  paper,  Whatman  No.  50,  7-cm  diam.,  hard.  Available:  1,  2. 
Filter  pump,  brass,  to  attach  to  water  faucet.  Available:  1,  2. 
Funnel  support,  wood,  supported  at  2  ends,  for  4  to  6  funnels.  Available:  1.  2. 
Graduated  cylinder.  10-ml  capacity.  Available:  1,  2. 
Hydrometer  jar.  Bouyoucos,  graduated  at  1,000  ml.  Available:  1,  2. 
Rubber  tubing,  pressure,  Y^-  and  %e_m-  inside  diameters.  Available:  1,  2. 
Sieve,  screen.  Standard  No.  35,  32-mesh  or  0.5  mm.  Available:  1,  2. 
Soil  hydrometer,  Bouyoucos  A.  Available:  1,  2. 
Solubridge  RD-26,  and  Pipette  Fill-type  Conductivity  Cell.  Available:  1,  2. 


[303  ] 


Test  tubes,  Pyrex  filtration,  with  side  tabulation,  18-  to  20-mm  outside  diameter. 

Available:  1,2. 
Test  tubes,  Pyrex,  with  rim,  150  mm  long,  20-mm  diameter.  Available:  1,  2. 
Thermometer  (see  above). 
Equipment  dealers 

1.  Braun  Corp.,  2260  E.  15th  St.,  Los  Angeles  21,  Calif. 

2.  Braun-Knecht-Heiman  Co.,  1400  16th  St.,  San  Francisco  19,  Calif. 

3.  Larvacide  Products,  Inc.,  1515  S.  3rd  St.,  San  Francisco,  Calif. 

4.  Neil  A.  Maclean  Co.,  Inc.,  184  S.  Alvarado  St.,  Los  Angeles  57;  470  8th  St., 
San  Francisco  3,  Calif. 

5.  Pike's  Peak  Greenhouses,  Inc.,  Colorado  Springs,  Colo. 

6.  Service  Chemical  Supply  Co.,  4937  Telegraph  Rd.,  Los  Angeles  22,  Calif. 


SOURCES  OF  FUNGICIDES  AND  CHEMICALS3 

Captan    (Orthocide  406,  Captan  50W).  N-trichloromethylmercapto-4-cyclohexene- 

1 :2  dicarboximide.  Available:  7,  25. 
Chloropicrin  (Larvacide).  Trichloronitromethane.  Available:  13,  15,  22. 
Copper  naphthenate.  Available:  4,  12. 
DD  mixture  (Dowfume  N,  Shell  D-D).  Dichloropropene,  dichloropropane  mixture. 

Available:  11,23. 
Demeton    (Systox).   0,0-diethyl  O-2-ethylmercapto-ethyl  phosphorothioate.   Avail- 
able: 6,  7,  8,  10,  14,  19,  20,  25. 
Dieldrin.    1,2,3,4,10.    10-hexachloro-6,    7-epoxy-l,4,4a,5,6,7,8,8a    octahydro-1, 4,5,8 

dimethanonaphthalene.  Available:  6,  7,  8,  10,  14,  19,  20,  25. 
Distilled  water.  Available:  1,  2,  18. 
Dura-K.  Potash  frit.  Available:  Glostex  Chemicals  Inc.,  3056  Bandini  Blvd.,  Los 

Angeles  23,  Calif. 
Ethylene  dibromide  (EDB,  Dowfume  W40  and  W85,  Bromofume  40  and  85,  E-D- 

Bee).  1,2  dibromomethane.  Available:  11. 
Ferbam    (Fermate,    Karbam,    Nu-leaf   Black,    Ferradow).    Ferric    dimethyldithio- 

carbamate.  Available:  8,  14,  17,  19. 
Formaldehyde.  Available:  4,  5,  6. 
Malathion.  S-(l  :2-dicarbethoxyethyl-0,0-dimethyl  phosphorodithioate) .  Available: 

6,7,8,10,14,19,20,25.  .    • 

Methyl  bromide,  gaseous  100  per  cent  (mixtures  containing  2  per  cent  chloropicrin 

sold  as  MC-2,  MBC  Fumigant,  Pestmaster  Soil  Fumigant).  Available:  11,  13,  15, 

22.  The  liquid  form  (Bromex,  Edco  MBX)   is  not  sold  in  California  for  use  on 

nursery  soils. 
Nabam   (Dithane  D-14,  Parzate  Liquid,  Ortho-Nabam) .  Disodium  ethylene  bisdi- 

thiocarbamate.  Available:  7,  8,  14,  17,  19,  21. 


'Sources  of  supply  are  listed  numerically  at  the  end  of  this  section;  numbers  refer  to  these 
sources.  Most  of  the  materials  are  also  obtainable  from  local  fungicide  or  insecticide  companies. 
Chemicals  mentioned  by  trade  names  in  the  text  are  listed,  with  some  sources  where  they  were 
available  when  ibis  manual  was  prepared.  No  endorsement  of  these  chemicals  or  sources  is  in- 
tended,  nor  is  criticism  implied  of  similar  products  or  sources  not  mentioned. 


!  304  I 


Nemagon.  1.2-dibromo-3-chloropropane.  Available:  23. 

Parathion.  O.O-diethyl  O-p-nitrophenyl  phosphorothioate.  Available:  6,  7,  14.  25. 

PCNB  (Mathieson  275.  Terraclor  I .  Pentachloronitrobenzene.  Available:  6,  16. 

V 

Potassium  chloride. solution.  Available:  9. 

100 

Puratized  Agricultural  Sprav.  Phenvl  mercury  triethanolammonium  lactate.  Avail- 
able: 20,  24. 

Semesan.  Hydroxymercuriochlorophenol.  Available:  14,  17. 

Sodium  hypochlorite  solution,  approximately  5  per  cent  available  chlorine.  May  be 
obtained  in  small  quantities  at  grocerv  stores  as  Clorox.  Purex,  etc.  Available: 
4,  5. 

Sodium  oxalate.  Available:  4,  5. 

Sodium  selenate  (mixture  with  2  per  cent  sodium  selenate  marketed  as  P-40). 
Available:  Plant  Products  Corp..  Blue  Point,  N.Y. 

Stoddard  solvent  (Standard  Thinner  No.  350).  Available:  3,  12. 

Thiram  (Arasan,  Tersan ) .  Tetramethylthiuram  disulfide.  Available:  14,  17. 

Triton  B-1956  Spreader.  Available:  21. 

Vapam  (N-869).  Sodium  N-methyl  dithiocarbamate  dihydrate.  Available:  25. 

V-C  13.  0-2.4-dichlorophenyl  O.O-diethyl  phosphorothioate.  Available:  Virginia- 
Carolina  Chemical  Corp..  Richmond  8.  Ya. 

Zineb  ( Parzate.  Dithane  Z-78).  Zinc  ethvlene  6zsdithioearbamate.  Available:  8,  14. 
17, 19,  21. 

Chemical  dealers 

1.  Alhambra  National  Water  Co.,  2217  Revere  Ave.,  San  Francisco,  Calif. 

2.  Arrowhead  and  Puritas,  Inc.,  1566  E.  Washington  Blvd.,  Los  Angeles,  Calif. 

3.  Bortz  Oil  Co..  9423  Exposition  Blvd.,  Los  Angeles,  Calif. 

4.  Braun  Corp..  2260  E.  15th  St.,  Los  Angeles  21,  Calif. 

5.  Braun-Knecht-Heiman  Co.,  1400  16th  St.,  San  Francisco  19,  Calif. 

6.  L.  H.  Butcher  Co..  3628  E.  Olympic  Blvd.,  Los  Angeles;  15th  and  Vermont  St., 
San  Francisco,  Calif. 

7.  California  Spray-Chemical  Corp.,  Lucas  and  Ortho  W  ay.  Richmond,  Calif. 

8.  A.  L.  Castle  Seed  Co.,  248  View  St..  Mountain  View,  Calif. 

9.  Central  Scientific  Co.  of  California,  6446  Telegraph  Rd..  Los  Angeles  22;   16 
Beale  St.,  San  Francisco  5,  Calif. 

10.  Chipman  Chemical  Co.,  Bay  Road,  East  Palo  Alto,  Calif. 

11.  Dow  Chemical  Co..  900  Wilshire  Blvd..  Los  Anseles:    350  Sansome  St..  San 
Francisco,  Calif. 

12.  Harshaw  Chemical  Co.,  3237  S.  Garfield  Ave.,  Los  Angeles,  Calif. 

13.  Larvacide  Products.  Inc..  1515  S.  3rd  St..  San  Francisco.  Calif. 

14.  Los  Angeles  Chemical  Co..  1960  Santa  Fe.  Los  Angeles.  Calif. 

15.  Neil  A.  Maclean  Co.,  Inc.,  184  S.  Alvarado  St..  Los  Angeles  57:  470  8th  St.,  San 
Francisco  3,  Calif. 

16.  Mathieson  Chemical  Corp.,  7183  E.  McKinley,  Fresno,  Calif. 

17.  McCrea  Seed  and  Chemical  Co..  Santa  Barbara.  Calif. 

18.  Mountain  Spring  W  ater  Co..  226  S.  Ave.  54.  Los  Angeles  42.  Calif. 

19.  Mover  Chemical  Co.,  P.  O.  Box  945.  San  Jose.  Calif. 


[305] 


20.  Niagara  Chemical  Division  of  Food  Machinery  Corp.,  P.  0.  Box  1589,  Richmond, 
Calif.;  206  Bassett  St.,  San  Jose,  Calif. 

21.  Rohm  and  Haas  Co.,  1404  Franklin  St.,  Oakland,  Calif. 

22.  Service  Chemical  Supply  Co.,  4937  Telegraph  Rd.,  Los  Angeles  22,  Calif. 

23.  Shell  Chemical  Co.,  1008  W.  6th  St.,  Los  Angeles;  100  Bush  St.,  San  Francisco, 
Calif. 

24.  James  A.  Southwick,  c/o  Food  Machinery  Corp.,  Riverside,  Calif. 

25.  Stauffer  Chemicals,  636  California  St.,  San  Francisco  8;   824  Wilshire  Blvd., 
Los  Angeles  14,  Calif. 

ACKNOWLEDGMENTS 

A  synthesis  such  as  this,  of  information  bordering  on  several  fields,  must  draw 
on  the  common  reservoir  of  knowledge  and  experience.  In  numerous  ways  this 
publication  represents  the  accumulated  experience  of  many  people  in  various  areas 
of  the  world.  Farm  advisors  and  growers  have  helped  check  the  validity  of  our  con- 
clusions in  local  practice,  and  have  suggested  modifications  of  them.  To  all  who  have 
so  generously  assisted  in  these  studies,  both  directly  and  through  their  published 
papers,  we  are  deeply  grateful. 

In  addition  to  those  specifically  mentioned  in  the  text,  we  are  especially  indebted 
to  M.  W.  Gardner,  W.  F.  Hiltabrand,  M.  R.  Huberty,  H.  J.  Ishida,  L.  J.  Klotz,  B. 
Lear,  O.  R.  Lunt,  J.  P.  Martin,  A.  O.  Paulus,  R.  L.  Perry,  C.  E.  Scott,  S.  A.  Sher, 
W.  C.  Snyder,  C.  Tasche,  J.  R.  Tavernetti,  R.  E.  Weidner,  and  S.  Wilhelm  for 
advice,  encouragement,  and  assistance  in  the  course  of  the  work  and  preparation  of 
the  manuscript.  Mrs.  L.  H.  Davis  prepared  the  line  drawings  and  R.  D.  Durbin  the 
graphs;  B.  B.  Markley  took  some  of  the  photographs.  Mrs.  Jane  Simonsen,  Mrs. 
Mary  Bogart,  Mrs.  Mary  Falzone,  and  Mrs.  Katharine  Baker  aided  in  the  prepara- 
tion of  the  manuscript  for  the  printer. 

The  material  in  each  section  was  prepared  by  the  named  authors,  read  and  dis- 
cussed by  others  of  the  group,  and  then  revised.  To  this  extent  the  information 
represents  the  collective  experience  and  concepts  of  the  ten  persons  directly  involved. 
Final  responsibility  rests,  however,  with  the  authors  of  each  section. 

Submitted  for  publication  June  10,  1955. 


[306] 


Acknowledgments,  306 

Actinomycetes,  21,  238,  242,  245,  254 

Adiantum  (maidenhair  fern),  215 

Adobe  soil,  86,  268 

Adopting  the  U.  C.  system,  1-2,  27,  79,  263-64 

Aeration  of  seedlings  and  damping-off,  43 

Aeration  of  soil,  10,  60,  87-89,  98-99,  120,  121, 
143,  259;  relation  to  fnmigant  diffusion, 
142-43;  to  microorganisims,  121,  140,  244; 
to  organic-matter  content,  98-99;  to  particle 
size,  98-99;  to  percentage  porosity,  87,  89, 
143;  to  pore  size,  143,  149;  to  steam  move- 
ment, 132-43,  149;  to  water  content,  87-89; 
to  water  movement,  65-67 

Aerobic  microorganisms,  244,  298 

Aeron  cover,  for  soil  treatment,  302 

African  violet,  see  Saintpaulia 

Aging,  of  seed,  21,  232;  of  soil  to  reduce  tox- 
icity, 10,  93,  95,  96 

Aglaonema,  226 

Aids  in  adopting  the  U.  C.  System,  1-2 

Air,  heat  conductivity  of,  299;  specific  heat  of, 
301 

Air-borne  spores,  46;  unimportant  in  Rhi- 
zoctonia  and  water  molds,  5,  250 

Aleurites  (tung),  257 

Alfalfa,  Rhizoctonia  on,  257 

Algae,  in  soil,  21,  238,  241;  on  containers,  19, 
211 

Alkali,  300 

Alkalinity,  soil,  87,  300 

Allerton  and  Ray  soil  mix,  94 

Allium  (onion),  258 

Aloe,  heat  treatment,  139,  226-27;  Pythium 
root  rot,  139,226-27 

Alternaria,  218,  229,  233 

Aluminum,  95;  irrigation  pipe,  129,  178,  195- 
96,  303;  specific  heat  of,  301 

Aluminum  sulfate,  76 

Amaryllis,  salinity  injury,  58 

Amino  acids,  51,  108,  245 

Ammonia,  anhydrous,  112;  aqua,  109,  112; 
toxicity,  112;  see  also  Ammonium 

Ammonifiers,  239-40,  245-46 

Ammonium,  105,  109,  126,  241,  243,  244,  245 
conversion  to  nitrate,  109,  113,  116-20;  pre- 
vented by  steaming,  113-14,  116-20 
difficulty  of  leaching  from  soil,  95,  99,  109 
effect  of  gypsum  on,  in  soil,  10,  96,  109 
injury  to  plants,  13,  14,  105,  109,  113,  252- 

54 
occurrence  in  field  soils,  109,  246 
release  from  organic  nitrogen,  13,  14,  53,  95, 
105,  115,  116-19;  not  prevented  by  steam- 
ing, 113-14 
retarded  movement   through   soil,    14,    109, 
122 


INDEX1 

role  in  soil  toxicitv,  94-96,  199 

toxicity,  9,  13-14,  79,  111-13,  121,  122,  199; 
control  by  omitting  organic  nitrogen,  13, 
115;  control  by  prompt  planting,  10,  95- 
96;  control  by  surface  application  of  or- 
ganic nitrogen,  14,  119;  control  by  use  of 
nitrate  fertilizers,  14,  111;  control  in  pot- 
ting-on,  119;  possible  cause  of,  253-51; 
sensitive  plants,  13,  96,  111,  112,  115 

See  also  Ammonia 
Ammonium  hydroxide,  112 
Ammonium  nitrate,  14,  53,  76,  78,  106,  1 12 
Ammonium  sulfate,  53,  76,  94,  106,  109,  111, 

112 
Anaerobic  microorganisms,  244,  298 
Anemone,  mosaic,  21,  233 
Anhydrous  ammonia,  112 
Animals,  245 
Antagonistic  organisms,  20,  21;  balance  in  soil, 

250-54;  relation  to  U.  C.-type  mixes,  24,  35, 

252-53;  use  to  prevent  soil  contamination, 

4,  20,  35,  250-53 
Antibiotics,   255-56,   298;    breakdown   in   soil, 

34;  produced  by  retardants,  24,  250-51,  300: 

toxic  to  crops,  24,  25,  251,  253;  use,  4,  34 
Antirrhinum,  see  Snapdragon 
Antiseptics,  use,  4,  34 
Aphelenchoides  (foliar  nematode),  20,  47,  139, 

220,  232-33 
Aphids,  235 
Apium,  see  Celery 
Apple,   crown  gall,   258;   fire  blight,  see   Fire 

blight 
Application  of  fertilizers 

dry,  11,  14,76-77,79 

inorganic,  see  Ammonium  and  Nitrate  nitro- 
gen 

liquid,  14,  27,  76,  78,  79;  proportioners  for, 
283,  302 

organic,  mixed  in  soil,  71-75,  122;  on  sur- 
face, 13-14,  118-19,  121,  122 
Aqua  ammonia,  109,  112 
Arachis  (peanut),  259 
Arasan  (thiram),  19,  43,  207,  209,  305 
Araucaria,  Rhizoctonia  top  rot,  38,  220 
Armillaria  root  rot,   135,   198;   infecton  from 

plant  parts,  260-61;  obtaining  stock  free  of, 

220 
Ascochyta,  233 
Aseptic  practices,  4,  34,  138 
Aspermy  virus,  221 
Asphalt  pavement,  41,  275 
Aster,  China,  see  China  aster 
Aster  yellows  virus,  232,  235 
Attitude  of  grower  toward  change,  27,  265 
Authors,  iv 
Autoclave    (cannery  retort)   for  soil  steaming 


Important  discussions  indicated  in  boldface  type. 


[307] 


Autoclave,  continued 

(type  9),  16,  17,  25,  129,  151,  162,  164,  166, 
174-75,  272;  disadvantages,  129,  158;  ther- 
mal efficiency,  129,  158;  unexpelled  air,  132, 
134,  147,  158',  174 

Avena,  139 

Avocado,  Phytophthora  root  rot,  45,  258 

Azalea,  12,  29,  69,  99,  120-21,  220;  chlorosis, 
107;  Ovulinia  flower  blight,  21,  30,  232, 
233,  234;  Rhizoctonia  decay  of  cuttings  and 
grafts,  38,  220;  root  injury  from  water,  poor 
aeration,  60,  99;  salinity  injury,  8,  60,  64 

Bacteria,  15,  18,  21,  197,  216,  221-22,  237-38, 
212,  245,  298;  ammonifying,  239-40,  245-46, 
254;  lethal  temperatures,  139;  longevity, 
243;  nitrogen-fixing,  244-45;  non-spore- 
forming,  113,  139,  245;  spore-forming,  113, 
139,  245,  254,  298;  sulfur,  246-47;  varia- 
bility of,  258 

Bacterial  blight  of  stock,  longevity  in  soil,  261 

Bacterial  fasciation,  220,  233,  258 

Bacterial  leafspot  of  delphinium,  46,  47 

Bacterial  soft  rot,  21,  35,  226-27,  233,  235; 
spread  by  fungus  flies,  227 

Bacterial  stem  rot  of  geranium,  261;  longevity 
in  soil,  261 

Baking  or  burning  of  soil  (type  17),  165,  166, 
177 

Balance  in  soil  microorganisms,  21,  25,  238- 
40,  250-54 

Balanced  soil  steaming,  130,  150,  155,  159-60 

Balled  stock,  31 

Bark,  ground,  12,  80,  97,  98,  99;  effect  on  soil 
leachability,  65-67;  phosphorus  content, 
99;  potassium  content,  99 

Base  exchange,  definition,  99;  of  fine  sand, 
99;  of  loam  and  clay,  99;  of  peat  moss,  99 

Bean,  treating  fields,  205;  Rhizoctonia  on,  43, 
205, 256 

Bean  straw,  268 

Bedding  plants,  29,  55,  84,  110,  126,  137,  203; 
sales  by  months,  30;  soil  mixes,  12,  69,  72, 
75,  79,  264-65;  U.  C.  system  for,  111,  113, 
119,264-65 

Beds,  chemical  treatment,  19,  200,  201,  203, 
204;  fertilizer  schedule,  77,  80-81,  120;  soil 
mixes  for,  80-81;  steaming,  131,  133,  134, 
162,  178-85,  192;  U.  C.  system  for,  12,  69, 
80-81,268 

Begonia,  79;  heat  treatment  of  tubers,  226; 
root-knol  nematode  on,  226;  salinity  injury, 
7,  9,  15,  53 

Belonolaimus  (sting  nematode),  261 

Benches,  126,  131,  210,  219,  250;  chemical 
treatmenl  of,  19,  40,  200,  204,  213,  215;  fer- 
tilize] s<  hedule  lor,  77,  80-81,  120;  soil  mixes 
for,  80-81;  steaming,  40,  132,  134,  159,  178- 
85,  211,  275;  U.  C.  s\stem  lor,  12,  69,  80-81, 
268 

Beneficial  soil  microorganisms,  238-40 


Bicarbonate  in  water,  64 

Bicvcle-wheel  cart  for  pots,  281-82 

Bin  and  potting  bench,  see  Combined  bin  ana 

potting  bench 
Bins,  134,  272-73 

Biological  control,  see  Controlled  colonization 
Bird-of-paradise  plant  (Strelitzia),  229 
Biuret  injury  from  urea  fertilizers,  13,  14,  78, 

79,  119 
Black  root  rot  (Thielaviopsis),  258 
Black  shank  of  cotton,  261 
Blood  meal,  13,  14,  53,  71-75,  76,  77,  79,  105, 

106,  121;  decomposition  in  soil,  115-19,  243 
"Blow  out"  of  steam  from  soil,   128,   149-50, 

152-53,   159-60,   183;   methods  for  prevent- 
ing, 130-31,  155 
Blow  torch,  bench  disinfestation,  19,  211 
Boiler,  steam 

capacity,  298 

cost,  calculation,  135-37 

efficiency,  thermal,  299 

fuel  for,  16,  162,  194-95 

horsepower  rating,  vs.  pounds  of  steam,  159- 
60,  193-94,  298;  vs.  kilowatt-hours,  159, 
300 

modified  hot-water  boiler,  192 

size  required,  1,  132-33,  159-60,  193-94,  298; 
calculation,  135-36 

sources,  302 

types,  159,  192,  302 
Boron    injury    to    plants,    not    prevented    by 

water  deionizers,  64;  related  to  water  supply, 

64;  required  by  plants,  89,  106 
Botrytis  cinerea,  5,  46,  47  (fig.  33);  aggravated 

by  salinity,  8,  9,  46,  55;  lethal  temperatures, 

139;  relation  to  Fusarium  wilt  in  aster,  49- 

50 
Box-type   soil    heaters,    with    electric    heating 

element    (type    14),    142,    165,    166,    176-77, 

179,  195,  303;  with  electrodes  (type  15),  165, 

166,    177,    179,    195;    with    induction    grid 

type  16),  165,  166,  177,  195;  see  also  Steam 

box 
Boxwood,  heat  treatment  of  plants,  226;  root- 
knot  nematode  on,  226 
Brassica,  see  Cabbage  and  Mustard 
Bromine  toxicity  from  soil  treatment,  10,  16, 

17,  18,  124,  199,201,  206,  208 
Broomrape,  longevity  in  soil,  261 
B.t.u.,  definition,  298 
Buffering  capacity,  299 
Bulbs,  217,  255 
Bur-clover  seed,  chemical  resistance,  203;  heat 

resistance,   139 
Buried    perforated     pipes    for    soil    steaming 

(type  20),    Hi,    128,    131,   132,    138,   148,   150, 
L52,  156,  165,  166,  178,  180-83,  LSI,  194 
Buried    tiles    lor   soil    steaming   (type   22),    16, 

131,    132,    1IH,    156,    165,    100,    178,   181,   183, 
IS5 
Bulla])  bags,  treatment,  212 


[308] 


Bushel,  301 

Butane,  fuel  for  boilers,  16,  162,  187,  192,  194 

Buttonweed  (Malva),  seed,  chemical  resistance, 

203;  seed,  heat  resistance,  139;  spotted  wilt, 

235 
Buxus,  226 

Cabbage,  43 

Cacti,  29,  37,  38 

Caladium,  bacterial  soft  rot,  226;  heat  treat- 
ment of  tubers,  226;  Sclerotium  rolfsii  on, 
139,  226 

Calcium,  13,  64,  70,  95,  99,  106,  109,  110;  avail- 
ability affected  by  microorganisms,  237, 
247;  essential  to  plants,  89,  106 

Calcium  bicarbonate,  63 

Calcium  carbonate,  70,  72-75,  91,  101,  106 

Calcium  chloride,  65-67 

Calcium  nitrate,  13,  14,  76,  78,  105-6,  112, 
119,  120,  265;  starter  solutions,  13,  14,  111- 
12,  254 

Calcium  phosphate,  14 

Calcium  sulfate,  14,  96,  101,  106,  113,  115;  see 
also  Gypsum 

Calendula,  42,  215 

California  climate,  effect  on  diseases,  29-30, 
233;  effect  on  salinity,  30;  relative  humidity, 
233 

California  "native"  plants,  37 

Calla,  263;  bacterial  soft  rot,  233,  234;  chemi- 
cal treatment  of  rhizomes,  20,  230;  harden- 
ing for  heat  treatment,  224;  hot-water 
treatment  of  rhizomes,  224,  229,  230;  isola- 
tion by  growing  in  pots,  234-35;  Phytoph- 
thora  root  rot,- 20,  229,  230,  233,  234-35; 
Rhizoctonia  on,  233;  spotted  wilt,  21,  235 

Callistephus,  see  China  aster 

Cal-mix,  93;  see  also  U.  C.-type  soil  mixes 

Calsoil  mix,  93;  see  also  U.  C.-type  soil  mixes 

Camellia,  69;  Rhizoctonia  on,  43,  220;  salinity 
injury,  58;  Sclerotinia  flower  blight,  21, 
232,  233,  234;  water  mold  root  rot,  220 

Canadian  peat,  see  Peat,  sphagnum 

Can  filler,  25,  26,  33,  166,  272-73,  280,  282, 
302 

Can  nursery  stock,  29,  31,  53,  126,  127,  137, 
163,  168,  171,  173,  196,  201,  210,  211,  267; 
fertilizer  schedules  for,  77,  80,  119;  soil 
mixes  for,  69,  72,  80,  267 

Canker,  stem,  34 

Cannery  retort,  see  Autoclave 

Cans  of  soil,  steaming  of,  142 

Capsella,  139 

Capsicum,  see  Pepper 

Captan  (Orthocide  406),  19,  43,  207,  209,  304 

Carbohydrate-nitrogen  status  of  plant,  253- 
54;  relation  to  damping-off,  5,  42 

Carbon,  237,  242-44,  246;  cycle,  246-47 

Carbon  dioxide,  6,  216,  241,  242;  exchange 
rate,  soil  to  air,  143;  in  air,  87,  246;  in  soil, 


87,   246,   259;    relation    to   development    of 
Rhizoctonia,  259 

Carbon/nitrogen  ratios  of  organic  matter,  re- 
lation to  bacterial  activity,  242-44;  relation 
to  fungus  activity,  242-44 

Carbon  tetrachloride,  223 

Carbonic  acid,  246;  relation  to  available  soil 
nutrients,  216 

Carborundum  powder,  232 

Carboxide,  216 

Carnation,  7,  32,  111,  201,  203,  205,  20K,  209, 
221,  234,  263;  ammonium  injury,  13,  111. 
113;  bacterial  wilt,  35;  bromine  injury,  10, 
16,  18,  95,  121,  199,  201,  206;  cultured-cut- 
ting technique  with,  20,  221;  mosaic,  234; 
Rhizoctonia  stem  rot,  43,  205,  207,  208; 
salinity  injury,  7,  55,  58;  U.  C.  system  for, 
268;  virus  diseases,  221,  232 

Castanea  (chestnut),  46 

Castor-bean  pomace,  13,  14,  76,  105,  115-19, 
121 

Cattleya,  90;  salinity  injury,  60 

CBP  55  (chloro-bromo-propene),  199 

Cedar,  Port  Orford,  46 

Celcure  wood  preservative,  216 

Celery,  25-26,  27,  111,  265-66;  aging  of  seed, 
21,  232;  ammonium  toxicity,  111;  aster  yel- 
lows, 235;  heat  treatment  of  seed,  51;  late 
blight,  21,  51,  218,  226,  232;  mosaic,  265 

Cellulose,  241,251,254 

Centipedes,  lethal  temperature,  139 

Centralized  soil  service,  27 

Ccratocystis  fimbria ta,  229 

Certification 
of  budwood,  31 

of  nursery  stock  in  California,  31,  48-49, 
129,  201;  and  soil  treatment,  48,  129,  201; 
and  U.  C.  system,  49;  "pinto  tag,"  48,  129, 
201 

Chalk,  see  Calcium  carbonate 

Chamaecyparis  (Port  Orford  cedar),  46 

Chelated  iron,  107 

Chemical   components,  of  dry   fertilizers,   76- 
77:  of  liquid  fertilizers,  76,  78;  of  U.  C.-type 
soil  mixes,  69-76;  see  also  Fertilizer  ingre- 
dients 

Chemical  residue  in  soil  after  treatment.  10. 
98,  199;  chloro-bromo-propene.  199;  ethv- 
lene  dibromide,  199,  206;  formaldehyde, 
201,  212-13:  kerosene,  190;  methyl  bromide, 
10,  16,  17,  18,  124,  199,  202,  208 

Chemical  treatment  of  propagative  material, 
20,  230,  232,  236;  effect  of  cracked  seed 
coats.  232;  ineffective  on  internal  parasites, 
41-42,  230 

Chemical   treatment  of  soil  (Section   11),  197- 
209;  also  frontispiece.  1,  11,  22.  2:>.    IS.  111. 
123,  210,  252,  251,  2S0 
aeration  of  treated  soil,  199 
best  treated  in  containers,  48,  126-27 
bulk  soil,  273 


[309] 


Chemical  treatment  of  soil,  continued 

causing  salinity,  53 

compared  with  steam,  1,  16,  18,  123-25,  210 

correct  dosage,  importance,  22 

cost,  197-98 

dosage.  249-50 

drenches,  19,  43,  48,  207,  209 

effect  on  microorganisms,  18,  120,  248-50 

effectiveness,  197,  201,  203,  204,  205 

equipment  for,  302 

eradicative  only  in  container  soils,  197 

ideal  chemical,  197-98 

importance  of  correct  dosage,  207,  249-50 

in  containers,  201-2,  272 

in  "pinto  tag"  certification,  48,  129,  201 

increased   growth   response   of   plants,    126, 
199,  248 

measurement  of  effectiveness,  125 

N  em  agon,  206,  305 

organic  matter,  effect,  197,  199 

preparation  of  soil,  198-99 

prior  to  placing  containers  on  it,  41 

selecting  one  to  use,  1,  208-9 

soil  moisture,  effect,  199 

soil  porosity,  related  to  fumigation,  89,  142 

soil  temperature,  effect,  148,  199 

time  required,  201-5 

toxic  residue  in  soil,  see  Chemical  residue 
in  soil  after  treatment 

treatment  of  soil  in  field,  18,  197,  201,  205, 
206,  208-9 

use  near  living  plants,  124,  125,  200 

V-C  13,  206,  305 

weed  control,  126,  197,  198,  200,  200-3,  204, 
208,  248 

when  best  used,  18,  197,  201,  208-9 

See  also  Chloropicrin;  DD;  Ethylene  dibro- 
mide;    Formaldehyde;    Methyl    bromide; 
Terraclor;  and  Vapam 
Chemicals,  sources,  304—6 
Chenopodium,  139 
Chestnut,  46 

Chickwced,  spotted  wilt,  235 
China  aster,  111,  215;  Botrytis  gray  mold,  49- 

50;  chemical  seed  treatment,  230,  232;  Fu- 

sarium  wilt,  5,  46-47,  49,  135,  218,  230,  232, 

257-58,  260;  Phomopsis  canker,  233;  Khizoc- 

tonia  on,  43,  49-50,  260;  spotted  wilt,  235; 

Stemphylium  leaf  spot,  233 
Chinese  evergreen  (Aglaonema),  226 
Chloranil  (Spergon),  230 
Chloride  in  water,  64 
Chloro-bromo-propene  (CHI*  55),  199 
Chlorophyll,  29«,  299 
Chloropicrin,  200-1,  205,  206,  208,  248,  272-73 

application,  16,  1H,  121,200-1,208-9 

comparison  with  methyl  bromide,  121,  198; 
with  steam,  121 

cost,  1H,  121,  137.  197-98 

effectiveness,  determination,  121;  in  crop  re 
fuse,   12  1,  200;  in  soil.   Mi,   IS,   12  1 


not  recommended  for  stacked  flats,  198,  201 

sorption  by  soil,  200 

sources,  304 

temperatures  for  use,  124,  201 

toxicity  to  crops,  124 

treatment,  of  benches  and  beds,  18,  201;  of 
bulk  soil,  18,201 

used  for  chrysanthemums  and  carnations, 
201,  208 

used  near  living  plants,  200 
Chlorosis,   of  gardenia,   control,   8,    107,   268; 

iron,    13,    107,    113;    result    of    biuret,    79; 

result  of  root-infecting  fungi,  15;  result  of 

salinity,  8,  55 
Choisya,  water-mold  root  rot,  36,  37,  220 
Chrysanthemum,  31,  32,  42,  203,  208,  219,  263, 

268 

Ascochyta  ray  blight,  233 

aspermy  virus  on,  221 

bacterial  fasciation,  220,  233 

bacterial  stem  rot,  35 

chemical  treatment  of  plants,  20,  232 

crown  gall,  220,  258 

culturing  technique,  20,  31,  221-22 

cutting  rots,  35 

foliar  nematode  on,  20,  232 

Septoria  leaf  spot,  233 

Verticillium  wilt,  7,  18,  31,  49,  201,  203,  219 

virus  stunt,  7,  51,  140 

viruses,  232 
Cineraria,  111 
Citrullus  (watermelon),  259 
Citrus,  psorosis,  232 
Clarkia,  13,  111 
Clay,  94,  97,  98-99,  100,  109,  142,  144,  161,  197, 

199,  264,  267-68,  276 

determination  in  fine  sand,  103-4 

disadvantages  of,  in  plant  culture,  12,  97; 
aeration  impaired,  98-99,  143;  cracks 
when  dry,  breaking  roots,  268;  drainage 
reduced,  65-67;  leaching  of  soluble  salts 
impeded,  65-67;  mixing  difficult,  98; 
movement  of  fumigants  and  steam  im- 
peded, 89;  shrinks  from  pots,  267;  toxic 
after  steaming,  93,  96;  variable  chemicallv 
and  physically,  97,  99 

effect  on  permeability  of  sand,  143 

tolerances  in  fine  sand,  103 
Clay  containers,  hot- water  treatment,  19,  53, 

171-73,  211-12;  salt  accumulation  on,  9,  53 
Clods,  see  Lumps  of  soil 
Clorox   (sodium- hypochlorite),  216,  221,  226, 

305 
Clover,  Persian,  root-knot  nematode  on,  259 
Coal,  fuel  for  boilers,  161-65,  176,  191 
"Cold  corners"  in  soil  steaming,  128,  134,  140; 

eliminating,  128,  134,  169,  177 
Colcus,  foliar  nematode  on,  220 
Collapse  of  plant,  from  damping-off,  35-36,  I  I, 

10;   from   root    rot,  36-37,    15;   from  salinity 

injury,  55,  59-60 


[310] 


Colonization,  controlled,  see  Controlled  colo- 
nization 

Comb  method  for  soil  steaming  (type  21),  183 

Combined  bin  and  potting  bench,  for  soil 
steaming  (type  3),  132,  148,  164,  166,  168-69, 
273,  278, 280-81 

Competition  escape  in  microorganisms,  aerial 
growth  by  Rhizoctonia,  251-52;  in  soil 
microorganisms,  259;  in  vascular  parasites, 
21,  238 

Competition  in  nursery  business,  means  of 
reducing,  32 

Competitive  retardant  microorganisms,  21,  24, 
238-40,  242,  300 

Components  of  U.  C.-type  soil  mixes,  see  In- 
gredients for  U.  C.-type  soil  mixes 

Compost,  91,  93,  97,  98,  99,  100;  cost,  85,  100; 
source  of  salinity,  30,  53;  source  of  toxicity, 
95;  source  of  variability,  10 

Composting,  avoided  by  U.  C.-type  mixes,  89- 
90,  93,  270;  disadvantages,  89-90,  100;  odors 
and  flies,  12,  90,  271;  reasons  for,  93-94,  109, 
243;  salinity  problem,  90;  scarcity  of  mate- 
rials, 90;  shrinkage,  89-90;  source  of  varia- 
bility, 10,  90;  weed  control  in,  126 

Computation  methods,  for  soil  particle  sizes, 
103-4;  for  steam  data,  132-33,  135-37,  159- 
60,  298-99 

Concrete,  particle  sizes  in,  99 

Concrete  mixers  for  soil  mixing,  25,  84,  187, 
265,  272-73,  276-79 

Condensation,  relation  to  steam/air  ratio,  146- 
51 

Condensation  zone  in  soil  steaming,  128,  146, 
149,  150,  152;  relation  to  efficiency  of  steam- 
ing, 149-52;  relation  to  steam/air  ratio,  146- 
51;  width  of,  150,  152 

Condensing  capacity  of  soil,  149-52,  154-55 

Conductance,  electrical,  as  measure  of  salinity, 
60-61;  method  of  measuring,  9,  14,  60-63, 
84,  299,  300,  303;  of  soil,  9,  61;  of  U.  C.-type 
mixes,  65,  70,  266;  of  water,  9,  14,  63 

Conduction  of  heat,  149,  177;  definition,  141, 
299;  effect  of  soil  moisture  on,  141-45;  fac- 
tors affecting,  in  soil,  141-42;  importance 
in  soil  heating,  150,  154-55;  relation  to 
porosity,  141-42;  relative  importance  for 
heat  and  steam,  142;  through  metal  con- 
tainer, 142 

Constant  water  level  culture,  relation  to  sa- 
linity, 63 

Container  culture,  advantages,  91,  234-35; 
earliest  example,  91-93 

Containers 

metal  or  plastic,  53,  211-12 

relation   to  spread  of  pathogens,  5,  39-40, 

210,217 
treatment  of  (Section  12),  210-16;  also  1,  11, 
19,  48,  111,  123;  required  when  soil  treated 
separately,  19,  22-23,  48,  125-27,  210,  273, 
279;  unnecessary  for  new  containers,  127; 


with  blow  torch,  19,  211;  with  copper 
naphthenate,  19,41,212,213-16,301;  with 
formaldehyde,  19,  212-13;  with  heat,  1, 
19,  211,  212;  with  hot  water,  19,  211-12; 
with  methyl  bromide,  19,  212;  with  steam, 
19,40,  133,  134,211-12 

Contamination  problem,  see  Recontamination 
problem 

Continuous-batch  steaming  equipment,  16, 
166,  169,  171,  174,  176 

Continuous  knife  injector  for  steaming  soil  in 
flats  (type  27),  148,  165,  166,  185-86 

Control  of  disease,  6,  48-49;  benefits  of,  7,  49- 
51;  multiple  controls  often  needed  in,  4,  219; 
must  mesh  with  nursery  practices,  4,  7; 
progress  in,  3-4,  34-35;  see  also  Chemical 
treatment  of  soil;  Containers,  treatment  of: 
Dry  source  of  heat;  Hot-water  treatment  of 
soil;  Pathogen-free  planting  stock;  Sanitary 
practices;  and  Steam  treatment  of  soil 

Controlled  colonization,  27,  250-54,  300;  effect 
of  pH  on,  251;  possible  future  program,  24, 
27,  254;  relation  to  U.  C.-type  soil  mixes 
24,  35,  252-54;  to  control  nitrogen  nutri- 
tion, 13,  25,  113-15,  116,  253-54;  to  retard 
pathogens,  4,  20,  24-25,  35,  250-53,  254 

Convection  of  heat,  149;  definition,  141,  148; 
importance  in  soil  heating,  143,  150,  154-55; 
relation  to  pore  size,  143;  relation  to  poros- 
ity, 143;  soil  factors  affecting,  142,  143-44 

Conveyers,  26,  168,  272-73,  278-80,  282 

Cooling  soil  after  steaming,  16,  134 

Copper,  95;  essential  to  plants,  89,  107 

Copper  naphthenate,  19,  41,  213-16;  sources, 
304 

Cordyline,  salinity  injury,  8,  9 

Coreopsis,  215 

Cork,  heat  conductivity,  299 

Corn, 240-41 

Corrosive  sublimate,  see  Mercuric  chloride 

Cotton,  205;  Fusarium  wilt,  261;  Rhizoctonia 
on,  205,  257;  Thielaviopsis  on,  258 

Cottonseed  meal,  13,  14,  76,  105,  115-19,  121 

Cottony  rot,  see  Sclerotinia  cottony  rot 

Covers,  fitting  to  glasshouse  benches,  180,  275; 
for  chemical  treatment  of  soil,  201-3,  208, 
302;  for  soil  in  steaming,  170,  178-80,  303 

Cowpea,  95 

Crop-antagonistic  microorganisms,  22,  238-40 

Crown  gall,  34,  218,  220,  239;  variability  in 
pathogenicity  of  bacteria,  258 

Cultural  practices,  and  disease  control  must 
mesh,  7;  evaluated  only  on  healthv  plants, 
51 

Culture-solution  growing  of  plants,  87 

Cultured-cutting  technique,  20,  221-22,  236 

Cuprinol  (copper  naphthenate),  19,  41,  213-16, 
304 

Cutting  rot,  4,  35,  37,  38;  see  also  Damping-off 

Cuttings,  rooted,  28,  79-80,  119,  217;  soil  mixes 
for,  71,  72,  73,  75 


[311] 


Cyanamide,  244 

Cyclamen,  Thielaviopsis  on,  258 
Cymbidium,  12;  salinity  injury  to,  8,  60;  U.  C- 
tvpe  soil  mixes  for,  268-69 

Dagger  nematode  (Xiphinema),  229 
Dahlia,  spotted  wilt  of,  221,  235 
Damping-off  of  seedlings   (Section   3),  34-51; 
also  4-6,  198,  202,  206 

aggravated  by  salinity,  5,  7,  9,  33,  42,  49-50, 
55,  266 

causes,  4-5,  33,  37 

control,  48-49;  see  also  Control  of  disease 
effect  of  carbohydrate  status  of  host  on,  5, 
42-43;  of  depth  of  planting  on,  5,  43;  of 
nitrogen  status  of  host  on,  5,  42-43;  of 
controlled  colonization  on,  24-25,  250-54; 
of  seed  vitality  on,  5,  43;  of  soil  moisture 
on,  5,  33,  36;  of  soil  temperature  on,  5,  6, 
43;  of  watering  on,  43 
factors  in,  5,  42-43 
infection  sites,  35 
losses  produced,  37 
not  restricted  to  seedlings,  6,  23,  36-37,  43- 

44,45 
recontamination  problem,  see  Recontamina- 

nation  problem 
relation  to  mechanization,  26-27,  32-33 
severity  related  to  host  susceptibility,  5,  42; 
to  inoculum  potential,  5,  42,  260;  to  soil 
treatment,  22,  248-50;  to  pathogen  viru- 
lence, 5,  6,  260 
types,  4,  35-38,  43,  249,  257 
Day  length,  relation  to  plant  distribution,  86 
DD  mixture,  application,  18,  206,  209;  effec- 
tiveness as  nematocide,  206;  sources,  304 
Dealers  of  equipment  and  materials,  304,  306 
Decomposition    of    organic   matter,    desirable 
before  use,  100-1;  effect  of  microorganisms 
on,  89,  95,  237,  240-44;  effect  of  soil  oxygen 
on,  240,  244;  effect  of  soil  temperature  on, 
240,  244;  see  also  Organic  matter 
Deionized  water,  for  leaching  soil,  15,  63-64; 
for  watering  plants,   15,  57,  63-64;  not  free 
of  boron,  64 
Delphinium,  111,  263;  aster  yellows,  235;  bac- 
teria] leaf  spot  (black  spot),  46,  47;  bacterial 
stem  rot,  218;  .sec  also  Larkspur 
Demcton    (Systox),    against    foliar    nematode, 

232;  sources,  304 
I)  ninific ation  process,  245 
Deposit  on  leaves,  salinity  injury,  9,  15,  53-54, 
64 

Dew  point .  definition,  146 

Di  an  thus,  see  Carnation 

Dial  hei  m\   for  soil  treatment,  190 

Dieffenbachia,   bacterial    leaf  spot,  220;    bac- 
terial   soft    rot,   35,   220,   226;   germinating 
cane,   227;    hardening  for   heal    treatment, 
223-24;  heal  treatmenl  of  cane,  220-27;  pre 
paring  cane  foi  treatment,  224;  Rhizoctonia 


on,  220,  257;  U.  C.  system  for,  267;  water- 
mold  stem  rot,  35,  220,  226 

Dieldrin,  used  against  fungus  flies,  227; 
sources,  304 

"Difficult"  crops,  32 

Diffusion  of  gases  through  soil,  89,  143,  146- 
49;  relation  to  porosity,  143,  148-49;  rela- 
tion to  temperature,  143-44 

Direct-tvpe  soil  heater  (type  15),  165,  166,  177, 
179,  195 

Disease  (Section  3),  34-51;  also  frontispiece, 
11,  300;  apparent  vs.  true  cause,  33;  develop- 
ment of  concept  of,  3-4,  34-35,  138;  elimi- 
nation, benefits  from,  7,  49-51;  factors  in, 
5,  22,  33,  49-51,  256;  importance  in  propaga- 
tive  material,  29;  importance  to  grower,  4, 
29,  49-51,  219;  relation  to  other  grower 
problems,  4,  7;  restricts  growth  potentiali- 
ties of  crop,  7,  49 

Dish  gardens,  fertilizer  and  soil  mixes  for,  81, 
84 

Distilled  water,  sources,  304 

Distribution  system  for  steam,  132,  136,  195- 
96;  aluminium  irrigation  pipe,  129,  191,  195; 
diameter  of  pipe,  191,  194,  195;  efficiency, 
299;  heat  loss  in,  194;  water  in  steam  lines, 
133,  152,  178,  196 

Dithane  D-14  (Nabam),  19,  207,209,  304 

Ditylenchus,  see  Stem  and  bulb  nematode 

Dolomite  lime,  70-75,  101,  106 

Don't  fight  'em,  eliminate  'em,  4,  7,  23 

Downy  mildew  of  snapdragon,  46,  47 

Drainage  of  soil,  11,  183;  effect  of  soil  condi- 
tioner on,  65-67;  improved  by  tiling,  183; 
in  relation  to  aeration,  60,  99;  in  relation  to 
salinity,  15,  53-54,  57,  64;  restriction  by 
container  boundary,  64,    87 

Dreft,  230 

Drenches,  fungicidal  and  fungistatic,  207,  209; 
captan  (Orthocide  106),  19,  43,  207,  209, 
304;  combination  of  materials,  207;  ferbam 
(Fermate),  19,  207,  209,  304;  nabam  (Di- 
thane D-14),  19,  207,  209,  304;  salvage  treat- 
ment, 207;  Semesan,  19,  207,  209,  305;  Ter- 
raclor  (PCNB),  19,  43,  207,  209,  305;  thiram 
(Arasan,    Iersan),  19,  43,  207,  209,  305 

Dripping  benches  from  steaming,  cause,  133, 
149 

Drum  soil  Heaters,  see  Horizontal  rotating 
(hum;  Oil-drum  method 

I)r\  fertilizers,  II,  14,76,77,  79,  80 

Dry  source  of  heat  for  soil  treatment,  frontis- 
piece, I,  16,  123,  162,  166, 176-77,  187,  189-91 
besl   with  moving  soil   mass,   1(5,  125-26,  163 
compared  with  steam,  125-26,  146 
disadvantages,  125,  146 
equipment  using,  176-77 
intense  heal  in  limited  area,  125 
objectives,  1  1 1 

temperature  and  time  required,  15 
temperature  in  moving  soil,  127 


[312] 


used  with  dry  soil,   144-46,   161,   176;   with 
moist  soil,  145,  161,  176 
Dump  soil,   methods   for  using,   84;   quantity 

used  for  bedding  plants,  84 
Dump  truck,  168 

Dura-K  potassium  frit,  76,  106,  304 
Duratex  cover  for  soil  treatments,  178,  303 
Dyes  indicating  liquid  fertilizer  injection,  76 

EDB  (ethylene  dibromide),  18,  198,  199,  206, 
209,  304 

Eddy  currents  of  steam  in  soil,  149 

Efficiency   calculations   for   soil   steaming,   299 

Eggplant,  25-26,  41,  264-65 

Egypt,  nurseries  in,  91 

Einheitserde  (standardized  soil),  94,  96 

Electricity,  power  for  boilers,  16,  162,  163,  164— 
65,  176,  177,  184,  185,  190,  192,  195 

Environment,  effect  on  disease,  5,  29-30,  42- 
43,  44,  49-50,  300;  effect  on  plant,  5,  42-43, 
86-89 

Environmental  tolerance  of  crop,  49 

Equipment,  fumigation  of,  138 

Equipment  for  mechanized  fertilizer  applica- 
tion, 76,  78,  283-84 

Equipment  for  mechanized  watering,  27,  283- 
84 

Equipment  for  planting,  Erdprinz  planter, 
282-83,  302;  machine  seeding,  25-26,  264, 
283;  pot  and  can  fillers,  25,  26,  33,  166,  168, 
272-73,  280,  302 

Equipment    for    soil    handling    (Section    17), 
275-82,  also  25-26 
bicycle-wheel  cart  for  pots,  281-82 
breaking  up  lumps,  98,  133,  161 
can  filler,  25,  26,  33,  166,  272-73,  280,  302 
concrete    mixer,    25,    84,    187,   265,    272-73, 

276-79 
conveyers,  26,  168,  272-73,  278-80,  282 
enclosed  storage  building,  163-66,  275-76 
flat  filler,  25,  26,  166,  168,  272-73,  279-80 
fork-lift  tractor,  25,  26,  166,   171,   174,  211, 

272-73,  277-79,  280-81 
mobile  bin  and  potting  bench  (type  3),  132, 

148,  164,  166,  168-69,  273,  278,  280-81 
moving  belts,  26,  168,  272-73,  278-80,  282 
pot  filler,  25,  168,  272-73,  280,  282,  302 
screen,  276-79 
shredder,  98,  276 
skip-load   tractor,  25,  84,   168,  272-73,  276, 

278-79,  280 
soil  treatment,  see  Equipment  for  soil  heat- 
ing, below 
steel  rollers,  25,  272-75,  277,  279,  282 
wooden  pallet,  22,  25,  26,  166,  171,  174,  211, 
212,  272-73,  276-77,  279,  280-81 

Equipment  for  soil  heating  (Section  10),  162- 
96;  also  1,  16 

adapting    batch    equipment    to    continuous 
operation,  16,  166,  169,  171,  174,  176 


autoclave  (type  9),  16,  17,  25,  129,  132,  134, 

147,  151,  158,  162,  164,  166,  174-75,  272 
baking  or  burning  (type  17),  165,  166,  177 
box,  electric  heating  elements  (type  14),  1  12, 

165,    166,   176-77,   179,    195,   303;    tubular 
version,  177 
box,  electrode  heating  (type   15;,   165,   166, 

177,  179,  195 

box,  induction  grid  (type  16),  165,  166,  177, 

195 
bulk  units,  15,  16,  167-77,  185-91 
buried   perforated   pipe   (type  20),    16,    128, 

131,  132,  138,  148,  150,  152,  156,  165,  166, 

178,  180-83,  184,  194 

buried  tile  (type  22),  16,  131,  132,  118,  156, 

165,  166,  178,  181,  183,  185 
combined  bin,  potting  bench  (type  3),  132, 

1  18,  164,  166,  168-69,  273,  278,  280-81 
containers,  treated  in,  162,  164-65 
continuous  knife  injector  for  flats  (type  27), 

148,  165,  166,  185-86 

deep  steaming  of  benches  or  beds,  131,  178, 

180-83 
efficiency  levels,  132 
electric  hot-plate  type  (type  32),  165,   189- 

90,  195 
free-flowing  vs.  superheated  steam,  129-30, 

156-59 
horizontal  tank  (type  13),  131,  164 
horizontal  type  with  removable  hood  (type 

10),  131,  164,  174-76,  195,272 
hot  water  (type  25),  16,  148,  165,  182,  184 
inverted   pan,   electric   (type   24),    148,    165, 

178,  180-81,  184,  195 
inverted  pan,  steam  (type  19),  16,  131,  132, 

138,  148,  165,  166,  178,  180-81,  184,  275 
mobile  bin  (type  2),  16,  132,  148,  164,  166, 

168,  193,  273,  280-81,  303 
mobile  units,  162,  273 
movable  Thomas  method  for  ground  beds 

(type  18),  148,  166,  180 
moving  rake  (type  23),   131,   148,   165,   166, 

178,  182-84 
multipurpose    tank    (type   7),   16,    131,    132, 

148,  164,  166,  171-73,  176 
oil-drum  type  (type  12),  131,  164,  176 
permanent  vs.  mobile  equipment,  163,  164- 

65 
rotating  drum,  external   flame,  batch  (typs 

35),  165,  191,  303 
rotating  drum,   external   flame,   continuous 

(type  31),  165,  190,273 
rotating    drum,    internal    flame    (type    30), 

165,  185,  187,  189-90,  194,  303 
rotating  drum,  knife  injector  (tvpe  28),  14S, 

165,  166,  185-87,  273 
rotating  screw,  electric  (type  33),  165,  189- 

90,  195;  external  flame  (tvpe  34),  165,  190 
rotating  screw,  steam  (tvpe  29),  16,  165,  187- 

88,  272,  303 


[313] 


Equipment  for  soil  heating,  continued 

Rudd  type  (type  1),  132,  148,  164,  166,  167- 

68,  180 
self-generating  types  of  steamers,  174-76,  184 
shallow  steaming  of  benches  or  beds,   131, 

178-80 
sources,  302-4 
spike  or  rake  type  (type  21),  135,  1 18,  150, 

165,  166,  178,  181,  183 
stationary  units,  162,  166-77 

stationary  vs.  moving  soil  mass,  16,  162-63, 

166,  185 

steam  box,  bulk  and  containers  (removable 
front)  (type  4b),  16,  17,  131,  132,  148,  156, 
164,  166,'  169-70,  303 
steam  box,  bulk  soil  (fixed  front)  (type  4a), 
16,  132,  148,  156,  164,  166,  167,  169-70,  176, 
273,  303 
steam-chemical  (type  26),  148,  165,  166,  184- 

85,  213 
steam-generating     equipment,     see     Steam- 
generating  equipment 
steam  plow,  182 
steam  vs.  dry  heaters,  125,  141 
table  for  selecting  suitable  type,  164-65,  274 
Thomas,  for  beds  (surface)  (type  18),  16,  131, 

132,  148,  163,  165,  166,  171,  178-80 
Thomas,   for   containers    (type  5),    16,    131, 

132,  148,  164,  166,  170-71,  179,  272 
tipping  steam  box,  Norwegian,  167 
vault   type   (type  6),    16,  25,   131,   132,   148, 
164,  166,  167,  171-72,  174,  272,277,  280-81 
vertical  cabinet,  electric  (type  11),  131,  174, 

175-76 
vertical    cabinet,    external    steam    (type    8), 

131,  132,  148,  164,  166,  173-75,  176 
vertical    cabinet,   self-generating   (type    11), 
131,  164,  174,  175-76,  195 
Equipment  for  soil  testing,  302 
Equivalents,  table  of,  301 

Eradication  of  pathogen,  difficult  in  field,  197; 
from   tools,   19,  22,  23,  40,  48,  201,  226;  in 
host  tissue,  20,  223;  see  also  Chemical  treat- 
ment   of    soil;    Containers,    treatment;    Dry 
source  of  heat;  Hot-water  treatment  of  soil; 
and  Steam  treatment  of  soil 
Erdalith  for  wood  preservation,  215,  216 
Erdprinz  planter  for  pots,  282-83,  302 
Erica,  see  Heather 

Esther  Read  daisy,  see  Chrysanthemum 
Ethylene   dibromide   (EDB),   application,    18, 
206,    209;    effectiveness,    198,    206;    residual 
toxicit)  to  plants,  199,  206;  sources,  304 
Eth) lene  oxide,  216 

Euphorbia  (poinsettia),  29,  257,  258,  266 
Evolution  ol  plants,  86,  91,  109 

lasuat ion,  bacterial,  220,  233,  258 
Ferbam  (Fermate),  19,  207,  209,  22!),  304 
Ferns,  foliar  nematode  on,  233;  salinity  injury, 

7,  58-59 


Fertility,  of  clays  and  composts,  99-100;  of  fine 

sand-peat  mixtures,  12,  99-100 
Fertilizer  burn,  aggravates  Botrytis  diseases,  46 
Fertilizer  ingredients  of  U.  C.-type  soil  mixes, 

69-76 

1,71-75,77,  78,  79,80,  81,  111 

11,71-75,  79,  80,  81,  111 

111,71-75,80,  111 

IV,  71-75,  77,  78,  79,80,  81,  111 

V,  71-75,  79,  80,  81,  111 

VI,  71-75,  80,  111 
cost,  71-75 
mixing,  70 

salinity  from,  15,  53-54,  57 

Fertilizers,  76-79,  87,  96 
deficit,  effect  on  plant,  88 
dry,  14;  application  methods,  11,  14,  76-77, 
79;   formulas    (fertilizers  VII-XI),  76-77; 
use,  examples,  79,  80 
evaluation,  51 

excess,  cause  of  chlorosis,  107 
inorganic,  see  Ammonium  and  Nitrate  ni- 
trogen 
liquid,  14;  application  during  watering,  14, 
27,  76,  78;  formulas  (fertilizers  L-l  to  L 
12),   76,   78;    proportioned   for   applying, 
283,  302;  use,  examples,  78-81 
mechanized  application,  78 
organic,  mixed  in  soil,  71-75,  122;  on  sur- 
face, 13-14,  118-19,  121,  122 
source  of  salinity,  9,  11,   15,  30,  53-54,  57, 
64-65,  70 

Fertilizing  equipment,  204,  302 

Fiberthin  covers  for  soil  treatments,  178,  303 

Fillers,  can,  25,  26,  38,  166,  272-73,  280,  282, 
302;  flat,  25,  26,  166,  168,  272-73,  277,  279- 
80;  pot,  25,  168,  280,  282,  302 

Fine  sand,  see  Sand,  fine 

Fir,  Douglas,  46;  bark,  65-67 

Fire  blight,  34,  258 

Fish  meal,  13,76,  115-19,  121 

Fittonia,  hardening  for  heat  treatment,  223- 
24;  heat  treatment  of  plants,  227-28,  229; 
Rhizoctonia  on,  220-21,  227-28,  267;  U.  C. 
system  for,  267;  water-mold  root  rot,  220- 
21,267 

Flash-flame  pasteurizer  for  soil  (type  30),  187— 
88,  190 

flash  steamers,  192,  302 

Flat -making  machine,  33,  276 

Flats,  126,  127,  150,  163,  196,  197,  201,  210, 
219,  250,  254;  chemical  treatment,  1,  19,  10, 
201,  203,  201,  208,  212-16;  fertilizer  schedule 
lor  plants,  79;  filler,  25,  26,  166,  168,  272- 
73,  279-80;  si/e,  279,  301;  soil  mixes  for,  12, 
69,  72,  75,  79,  264-65;  stacking  for  soil  treat- 
ment, 131,  173,  189,  201-4,  211:  steam  treat- 
ment, 19.  10,  133.  13  1,  162,  163,  170,  171, 
173,  211;  transporting,  22,  25,  26,  166,  171, 
171,  21 1.  212,  272-73,  276-77.  279,  280-81 


[314] 


Flies   from   compost    piles,    12,   90,   271;    from 

organic  nitrogen,  122 
Floricultnral  plants,  28 
Flower  blights,  34 
Foliage  plants,  20,  29,  32;  U.  C.  system  for,  20, 

29,  32,  266-67;  utilize  ammonium,  13 
Foliar  feeding,  89 
Foliar  nematode  (Aphelenchoides),  20,  47,  220, 

233;  chemical   treatment  of  plants  against, 

232;  lethal  temperatures,  139 
Fork-lift    tractor,   25,   26,    166,    171,    174,    211. 

272-73,  277-79,  280-81 
Formaldehyde.  200,  203-4,  208 

dilute  method  for  soil,  204,  208;  relation  to 
inoculum  potential,  42,  204,  260 

effectiveness,  203 

paraformaldehyde  formation,  19,  212-13 

sources,  304 

steam-formaldehvde  for  soil  treatment,  148, 
184;  for  glasshouse  cleanup,  213 

toxicity  to  plants.  204 

treatment,  of  containers,  19,  212-13;  of  floor, 
18,  22,  134,  166,  199;  of  planting  material, 
20,  228,  230;  of  soil,  drench,  41,  148,  204, 
208;  of  tools,  19,  22,  23,  40,  213,  226 
Formalin,  see  Formaldehyde 
Fragaria  (strawberry),  220 
Frankincense  trees,  91 
Free-flowing  steam  for  soil  treatment,  15,  129, 

135,    157-58,    164-65,    167-74,    178-84,    187, 

191-92,  196,  300;  advantages,  129;  heat  con- 
tent, 157-58;  pressure  in  mains,  129 
Freesia,  mosaic,  21,  233 
Frit,  potash,  76,  106 
Frost,  relation  to  plant  distribution,  86 
Frozen  soil,  cause  of  uneven  steaming,  134 
Fuchsia,  foliar  nematode  on,  220 
Fuel  for  steam  boilers,  16 

amount  required  for  soil  treatment.  135-36 

butane,  16,  162.  187,  192,  194 

coal,  164-65,  176,  194 

cost,  136 

electricity,    16,    162,    163,    164-65,    176,    177, 
184,  185,  190,  192,  195 

heating  value,  299 

kerosene,  187,  194 

natural  gas,  16,  135,  136,  162,  163,   164-65, 
174,  176,  185,  187,  190,  191,  192,  194 

oil,  16,  136,  162,  163,  164-65,  174,  176,  185, 
190,  192,  194 

propane,  162,  192,  195 
Fumigants,  see  Fungicides  and  Xematocides 
Fungi,  15,  18,  197,  198,  200,  204,  206,  221,  237- 

38,  242,  254,  299;  longevity,  199,  243:  nema- 

tode-trapping,  240;   variability,  255-58;  see 

also  Damping-off 
Fungicides,  259,  299 

captan  (Orthocide  406),  19.  43,  207,  209,  304 

Celcure,  216 

chloropicrin,  see  Chloropicrin 


copper  naphthenate  (Cuprinol),  see  Copper 
naphthenate 

Erdalith,  215,  216 

ethylene  oxide  (Carboxide),  216 

ferbam  (Fermate;,  19,  207,  209,  228,  301 

formaldehyde,  see  Formaldehyde 

mercuric  chloride,  20,  230,  232 

methyl  bromide,  see  Methyl  bromide 

nabam  (Dithane  D-14),  19,  207,  209,  30! 

New  Improved  Ceresan,  20,  230 

Puratized  Agricultural  Spray,  228,  305 

Semesan,  19,  207,  209,  305 

sodium   hypochlorite   (Clorox,   Purex),  216, 
221-22,  226,  305 

sources,  304-6 

Spergon  (Chloranil),  230 

sulfur,  246-47 

Terraclor  (PCNB),   19,  43,  205,  207,  208-9, 
305 

thiram  (Arasan,  Tersan),  19,  43,207,  209,  305 

Yapam,  18,  204-5,  208,  305 

Wolman  salts,  215,  216 
Fungistatic,  299 

Fungus  flies,  control,  227;  spread  soft-rot  bac- 
teria, 227 
Fusarium,  198,  220,250 

basal  rot,  227-28 

cortical  root  rot,  229,  256 

cortical  stem  rot,  256 

effect  of  soil  temperature,  260 

lethal  temperatures,  127,  139 

longevity  in  soil,  261 

saprophytes  and  parasites  in  soil,  238 

saprophytic,  256 

survival  in  soil,  261 

variability,  256-58 

vascular  wilts,  139,  256-57 

wilt,  favored  by  nematodes,  261 
Fusarium  wilt  of  China  aster,  5,  135,  230,  232, 

257-58;  conditions  favoring,  47,  49,  260;  in- 
oculum potential,  260;  life  history  of  causal 

fungus,  218;  relation  to  Botrvtis  crown  rot, 

49;   relation   to  Rhizoctonia  crown   rot,  49; 

soil  temperature,  260;  symptoms,  46-47 

Galvanized  nails,  corrosion  bv  copper  naph- 
thenate, 213 

Gardenia,  35,  69,  268;  chlorosis,  8,  107,  268; 
salinity  injury,  8,  55,  59-60;  U.  C.  system 
for,  268 

Gas,  natural,  fuel  for  boilers.  16,  135.  136, 
162,  163,  164-65,  174,  176.  185.  187.  190.  191. 
192,  194 

Gases,  viscosity  related  to  temperature.  143 

Geranium.  29,  32,  35,  215-16;  bacterial  leaf 
spot  and  stem  rot  of.  21.  35,  261:  cultured- 
cutting  technique  with,  20,  221;  mosaic  of. 
234 

Gerbera,  heat  treatment  of  plants.  227;  root- 
knot  nematode,  227 

German  peat,  see  Peat,  sphagnum 


[315] 


Germination 

effect  of  pelleting  on,  25,  283;  of  salinity  on, 

9,  42,  55 
in  mechanization,  26-27,  264 
rate,  seed,  82;  effect  of  soil  toxicity  on,  94 

Gladiolus,  223,  263;  effect  of  soil  temperature 
on  heat  tolerance  of  cormels,  224;  heat  treat- 
ment of  cormels,  224;  presoaking  cormels 
before  heat  treatment,  224;  Rhizoctonia  on, 
257;  yellows  disease,  139 

Glossary  of  terms  used,  298-301 

Glycine  (soybean),  257 

Gossypium  (cotton),  205,  257,  258,  261 

Graft  failure,  Thielaviopsis,  258 

Gravel,  97,  98,  99,  100,  103-4 

Gray  mold  (Botrytis),  5,  9,  46-47,  49-50,  139 

Grower  experience  with  U.  C.  system  (Section 
16),  263-70;  also  1;  bed  flower  crops,  208; 
bedding  plants,  264-65;  bench  flower  crops, 
268;  can -grown  woody  stock,  267;  cymbi- 
diums,  268-69;  foliage  plants,  266-67;  land- 
scape application,  269-70;  pot  plants,  266- 
67;  vegetable  transplants,  265-66 

Grower  "secrets,"  49 

Growing-on,  soil  mixes  for,  71,  73,  74 

Growth  regulators,  51 

Gypsum,  source  of  calcium,  71-72,  101,  106; 
used  to  reduce  soil  toxicity,  10,  96,  109;  see 
also  Calcium  sulfate 

Hardpan  in  soil,  and  salinity,  57 

Harrow   method  of  soil    steaming,  see   Spike 

method 
Harrow-type  electrode  heater  (type   15),   165, 

166,  177,  179,  195 
Haworthia,  heat  treatment  of  plants,  139,  227; 

Pythium  root  rot,  139,  227 
Heat,     definition,     141;     differentiated     from 

steam,  142 
Heat  capacity,  definition,  141,  299,  301 
Heat  requirement  of  soil,  299;  of  water,  299 
Heat    sterilization,    development    of,    138;    of 

containers,  1,  19,211,212 
Heat-tolerant  plant  parts  for  treatment,  223- 

24 
Heat  transmission 

compared  with  steam  movement,  1  11-45 

conduction,  141-42,  111,  150 

convection,  141,  142,  143-44,  148,  150,  151-55 

radiation,  141,  142,  144,  150 

rate,  in  dry  soil,  144-45;  in  moist  soil,  144-45 

) elation  to  compaction,  130,  134,  112;  to  or- 
ganic mattei  content,  112;  to  particle  size, 
I  12;  to  soil  porosity,  I  11-12;  to  soil  mois 
line-.  111    1") 

Stead)  and  misleads  states,  145-46,  150 

through  pine  logs,  151-52 
I  leal  treatment 

of     planting    material,     138,    236;    against 
microdrganisms,      139,     223-31;     againsl 


viruses,  221;  See  also  Hot-water  treatment 
of  planting  material 
of  soil  (Sections  8,  9),  123-61;  also  11,  210;  see 
also  Dry  source  of  heat  for  soil  treatment; 
Hot-water  treatment  of  soil;  and  Steam 
treatment  of  soil 

Heather  (Erica),  chlorosis,  107;  cutting  rot, 
45;  Phvtophthora  root  rot,  7,  45,  49,  218-19, 
220,  258;  Rhizoctonia  stem  rot,  220 

Heating  of  soil,  effect  on  organic  nitrogen 
breakdown,  13,  53,  105,  112,  115-19,  120 

Hedera  (ivy),  220-21,  267 

Helichotylenchus  (spiral  nematode),  229 

Helminthosporium  cactorum,  38 

Herbaceous  ornamentals,  29 

Heterodera,  see  Potato  root  nematode  and 
Root-knot  nematode 

High  vs.  low  pressure  steam  for  soil  treatment, 
191-92 

Hippeastrum  (amaryllis),  58 

"Hoddesdon  pipe"  for  soil  steaming,  180; 
winch-drawn,  182 

Home-yard  planting,  fertilizer  schedule,  80- 
81;  soil  mixes,  81,  84;  steaming  soil,  135,  193 

Hoof  and  horn  meal,  13,  14,  53,  70-75,  76,  77, 
79,  91,  105,  106,  110,  111,  112,  113,  115-19, 
121,242,243 

Horizontal  rotating-drum  soil  heaters,  exter- 
nal flame,  batch  type  (type  35),  165,  191,  303; 
external  flame,  continuous  type  (type  31), 
165,  190,  273;  internal  flame  (type  30),  165, 
185,  187,  189-90,  194,  273,  303;  knife-injec- 
tor type  (type  28),  148,  165,  166,  185-87,  273 

Horizontal  rotating-screw  soil  heaters,  electric 
(type  33),  165,  189-90,  195;  external  flame 
(type  34),  165,  190;  steam  (type  29),  16,  165, 
187-88,  272,  303 

Horizontal  tank-type  soil  steamer  (type  13), 
131,  164,  176 

Horizontal-type  steamer,  removable  hood 
(type  10),  131,  164,  174-76,  195,  272 

Hormone  solutions,  relation  to  spread  of 
pathogens,  5,  22,  38 

Horsepower  rating  of  boiler  vs.  pounds  of 
steam,  159-60 

Hose  nozzle  and  spread  of  pathogens,  5,  23,  24, 
38-40 

Host,  299,  300 

Host  range  of  pathogens,  6,  256-59;  variability 
in.  256-59 

Hot-plate  method  of  soil  heating  (type  32), 
165,  189-90,  195 

Hot-water  treatment  of  containers,  methods, 

19;  salt  removal,  19,  211-12 
Hot-water  treatment  of  planting  material.  20, 
Hi  I  65,223-30,236 
application  to  specific  crops,  226-30 
breaking  dormanc  s  of  stock.  227.  229,  230 
conditioning  the  material,  20,  223-24 
containers  lor  material.  225 
cooling  the  material,  225-26 


[  316  ] 


drying  the  material,  226 

equipment,  302 

eradication  of  pathogens,  20,  223 

methods,  223-26 

multipurpose  tank,  171-73,  224 

not  protective  against  reinfection,  223 

preparing  the  material,  20,  224 

presoaking  the  material,  224 

selecting  material  to  treat,  223 

storing  the  material,  226 

temperature-time  relation,  20,  139,  225 

treatment  tanks,  224-25 

value,  223 

Hot-water  treatment  of  soil,  123;  compared 
with  steam,  126,  146-48,  152,  156-57,  184; 
disadvantages,  126,  146;  equipment  for 
heating  by  steam  (type  25),  16,  148,  165, 
182,  184;  excessive  water,  126,  184;  heat  con- 
tent of  hot  water,  146,  156-57;  heat  transfer 
involved,  146,  156-57;  salt  removal,  126,  146, 
184;  use  on  propagating  sand,  126,  146,  184 

Humidity,  atmospheric,  146-47;  effect  on  dis- 
eases, 29-30,  42-43;  on  salinity  injury,  11, 
15 

Humus,  99 

Hutchings  method  for  soil  heating  (type  32), 

165,  189-90,  195 
Hybrid  seed,  32 

Hyacinth,  stem  and  bulb  nematode  on,  258 
Hydrangea,  blue  vs.  pink,  76 
Hypericum,  139 
Hypnum  peat  moss,  69,  104 

Iceland  poppy,  111,  215 

Immersion-type  soil  heater  (type  14),  142,  165, 

166,  176-77,  179,  195,  303 
Increase  block,  222,  235-36 

Increased    growth    response    from    soil    treat- 
ment, 126,  199,  248 
Incubation  period  of  disease,  232,  234 
Indirect-type  soil   heater  (type   14),   142,   165, 

166,  176-77,  179,  195,  303 
Induction-grid  type  soil  heater  (type  16),  165, 

166,  177,  195 
Infected  seed  or  stock,  43,  44,  260;  increase  of 

inoculum  potential,  260;  means  of  selecting 

virulent  strains,  6,  260 
Infection,  299;  by  Rhizoctonia,  38 
Infest,  definition,  299 

Infrared  lights  for  soil  heat  treatment,  190 
Ingredients  for  U.  C.-type  mixes,   12-13,  69- 

76,  96-107 

aeration,  97,  98-99 

availability,  12,  97,  98,  103 

characteristics,  101-7 

chemical,  69-76,  97,  98,  101,  105-7;  see  also 
Fertilizer  ingredients 

cost,  97,  100 

criteria  for  selection,  96-107 

ease  of  mixing,  83-84,  97,  98,  105,  133,  276- 
79 


fertility,  97,  99-100,  105-7 

moisture  retention,  12,  69,  87,  97,  100,  270 

physical,  69,  101-5;  see  also  Peat,  sphagnum; 
Redwood;  Rice  hulls;  and  Sand,  fine 

proportions,  69-76 

resistant  to  leaching  of  nutrients,  12,  97,  99 

selection,  96-107 

shrinkage,  12,  80,  85,  89-90,  97,  100-1,  241 

soil  used,  low  in  organic  matter,  103 

sources,  85,  97-98,  101-3,  275 

stability  to  steaming  or  fumigation,  9-10, 
11,  12,  15,  90,  93,  96,  97-98,  124,  129,  140, 
199,270 

uniformity,  12,  97,  98 

weight,  12,69,  80,97,  100 
Injector,  for  applying  fertilizers,  283,  302;  for 

applying  soil  fumigants,  200,  201,  206,  302 
Inoculation  of  soil,  with  ammonifying  micro- 
organisms,   13,    113-15;    with    antagonistic 

microorganisms,   4,   20,   24-25,    35,   250-54; 

with  nitrifying  bacteria,  13,  25,  113-15,  116, 

120, 253-54 
Inoculum  potential,  5,  42,  204,  260,  300 
Insect  screens  on  seedbeds,  235 
Insecticides,  18,  20,  206,  227,  232,  304,  305 
Insects  and  mites,  15,  18,  21,  138,  197,  198,  200, 

204,  206,  248,  275;  lethal  temperatures,  127, 

139 
Inserts  for  flats,  25,  31,  127 
Introduction  of  new  disease,  importance,  20, 

218;  see  also  Spread  of  microorganisms 
Inverted  pan  for  soil  steaming  (type  19),  16, 

131,  132,  138,  148,  165,  166,  178,  180-81,  184, 

275;   electric  type  (type  24),   148,   165,   178, 

180-81,  184,  195 
Ipomoea    (morning-glory    and    sweet    potato',, 

221,257 
Iris,  139,  260 

Iron,  95;  availability  affected  by  microorgan- 
isms, 237,  247;  chelate  for  chlorosis,  107,  268; 

chlorosis,   13,   107,   113;   essential   to  plants, 

89,  107 
Isolation  house,  21,  23,  24,  234-35,  261 
Ivy,  bacterial  leaf  spot,  221;  Rhizoctonia  on, 

220-21;   U.  C.  system  for,  267;   water-mold 

root  rots,  220-21 


John  Innes  composts,  10,  86,  91,  93;  contribu- 
tion to  nursery  soils,  10,  91,  93,  110;  disad- 
vantages, 93,  96;  sold  on  market,  94 

John  Innes  high-rate  soil  steamer  (type  1), 
167-68 


Kentia  palms,  284 
Kerosene,  fuel  for  boilers,  187,  194 
Kilowatt-hours,  159,  300 
Klamath-weed  seed,  heat  resistant,  139 
Krillium,    effect    on    leachability   of   soil,    15, 
65-67 


L  317  J 


Labor,  cost  rising,  32,  96,  196,  271;  reduced  by 

U.  C.-type  soil  mix,  10,  84,  89-90,  100,  270 
Lactuca  (lettuce),  95,  112 
Lamb's  quarter  seed,  heat  resistant,  139 
Landscape  use  of  U.  C.  system,  269-70;  soil 

fumigation  in  field  plantings,  269-70 
Land  value  rising,  effect  on  nurseries,  31 
Larkspur,  215 
Lathyrus  (sweet  pea),  230 
Leachability  of  U.  C.-tvpe  soil  mixes,  65-67, 

99 
Leaching 

losses  of  soil  nutrients,  65-67,  99,  105-7 

to  reduce  salinity,  9,  11,  14-15,  183;  effec- 
tiveness related  to  water  salinity,  9,  14, 
63;  relative  leachability  of  various  soil 
mixes,  15,  65-67;  with  nutrient  solution, 
65 

to  reduce  toxicity,  10,  95,  96 
Leaf    burn,    from    ammonium,    13,    111,    113; 

from  biuret,  79;  from  salinity,  8-9,  42,  55- 

56,  58 
Leaflet  on  U.  C.-type  soil  mixes,  68 
Leaf  mold,  86,  97,  98,  99,  115-19,  242,  262,  267, 

268 

aeration  when  decomposed,  99 

cost,  85,  100 

scarce  in  California,  12,  90,  98,  100 

shrinkage,  85,  90 

source  of  salinity,  9,  15,  30,  53-54,  90,  264; 
of  toxicity,  95,  108;  of  variability,  10,  89, 
93 
Leaf  spots,  34 
Legumes,  77,  78,  245 
Lesion  nematodes  (Pratylenchus),  47,  227,  229; 

lethal  temperatures,  139 
Lettuce,    ammonia    toxicity,    112;    manganese 

toxicity,  95 
Light  on  plant,  29,  51,  259;  relation  to  damp- 

ing-off,  5,  43;  relation  to  plant  distribution, 

86 
Light  soil   mix,  93;  see  also   U.   C.-type  soil 

mixes 
Lignin,  254 
Lily    (Lilium),   86;    Fusarium   basal    rot,   227, 

229;   root   development   in    U.   C.-type   soil 

mix,  83  (fig.  61);  treatment  of  bulbs  against 

nematodes  and  Rhizoctonia,  227-28,  229 
Lima   bean,  Rhizoctonia  on,  257 
Lime,  70.  91,  101,  115,  120;  calcium  carbonate, 

70,    72-75,    91,    101,    106;    dolomite,    70-75, 

101,  100;  ovstei shell,  106 
Liners,  soil  mixes  for,  71,  72,  73,  80 
Lining-out  slock,  29 
Liquidanibar ,  chlorosis,  107 
Liquid  fertilizers,  14,  27,  76,  78-79,  106;  pro 

portioners  for  applying,  283,  302 
Little-leaf  of  peach,  248 
"Living  uilh"  a  disease,  7,  49 
Loam,  15,  65-67,  91,  97,  98,  99,  100,  1  12,  1  13, 

I  11,  I")  I  55 


Lobelia,  111,215 

Lobularia  (sweet  alyssum),  13,  111,  112,  113, 
264 

Loganberry,  crown  gall,  258 

Longevity  of  organisms  in  soil,  261 

Long  pipe  for  soil  steaming,  180,  184;  winch- 
drawn,  182,  184 

Lotus  strigosus  seed,  heat  resistant,  139 

Lumps  of  soil,  equipment  for  breaking  up, 
98,  133,  161;  not  formed  by  fine  sand,  98, 
133,  161,  276;  relation  to  chemical  treat- 
ments, 199;  relation  to  steaming,  15,  127, 
133,  134,  140,  149,  160-61,  190 

Lycopersicon,  see  Tomato 


Machinery,  disinfestation,  201,  212 
Magnesium,  64,  95,  99,   109,   110;  availability 

affected    by   microorganisms,    237,   247;    es- 
sential   to   plants,   89,    106;    from   dolomite 

lime,  13,  70 
Magnesium  bicarbonate,  63 
Magnesium  carbonate,  106 
Magnesium  sulfate,  94,  177 
Maidenhair  fern,  215 
Maintenance  of  pathogen-free  planting  stock, 

21,  226,  234-36;  by  selecting  growing  area, 

21,  233;  of  chrysanthemums,  31 
Majestic  daisy,  see  Chrysanthemum 
Malathion,  against  fungus  flies,  227;  sources, 

304 
Mains  (apple),  258;  see  also  Fire  blight 
Malva  (buttonweed),  139,  203,  235 
Manetti,  see  Rose 

Manganese,    availability    affected    by    micro- 
organisms, 247-48;   essential   to   plants,   89, 

106-7;  role  in  soil  toxicity,  9,  95,  98 
Manure,  97,  98,  115-19,  198,  241-42,  262,  264, 

265,  267,  268,  271 

aeration,  when  decomposed,  99 

competition,  12,  90 

cost,  85,  100 

package  trade,  90 

poor  source  of  nutrients,  99,  100;  of  organic 
matter,  89,  100 

shrinkage,  85,  89-90,  100,  241 

source,  of  salinity,  9,  30,  53-54,  90,  116;  of 
toxicity,  95,  108;  of  variability,  10,  89,  93 
Maple  leaves,  100 
Maranta,  salinity  injury,  8-9 
Marguerite,  see  Chrysanthemum 
Marigold,  see  Calendula  and  Tagetes 
Market,  distance  from,  31-32;  expanding  in 

California,  31-32;  lor  bedding  plants,  30-31 
Materials,  sources,  302-6 
Matthiola,  see  Stock 
Meadow    nematode    (Pratylenchus),     17,    139, 

227,  229 
Measures,  table  of,  301 
Mechanical    applicators    of    fumigants,    201. 

20  1  -5,  200 


[318] 


Mechanization  in  growing  (Section  17),  271- 
84;  also  frontispiece,  l',  25-27,  32,  33,  266 
adaptability  of  U.  C.  system  to,  25,  90,  94, 

270,271 
adoption  when  moving,  31 
advantages  of  sloping  land  for,  32,  274 
applied    to    new    nursery,    274-75;    to    old 

nursery,  274 
bicycle-wheel  cart,  281-82 
disease  control  required,  26-27,  32-33 
fertilizing,  78,  204,  283-84,  302 
filling  containers,  25,  26,  33,  166,  168,  272- 

73,^  279-80,  282,  302 
flat-making  machine,  33,  276 
flow  diagrams,  1,  272-73 
germination  in  covered  flats,  26-27,  264 
glasshouse  arrangement  and  design,  275 
laird  slope  in  mechanization,  32,  274 
machine  planting,  25-27,  264,  283,  302 
mixing  and   screening  soils,   25,   83-84,   97, 

98,  187,  265,  272-73,  276-79 
mobile  bin  and  potting  bench  (type  3),  132, 

148,  164,  166,  168-69,  273,  278,  280-81 
paving  the  yard,  275 
planning  layout,  31,  274-84 
planting,  25-27,  264,  283,  302 
preparing  soil  mixes,  83-84,  105,  133,  276 
processing  and  stockpiling  materials,  275-77 
requirements  to  make  possible,  26-27,  32-33 
seeding  in  place,  25-26,  264,  283,  302 
segregation  of  operations,  21,  163,  166,  276 
storage  of  components,  275-76;  of  soil  mixes, 

13,  25,  71-75,  276 
transplanting,  25,  81,  264,  282-83 
transporting    containers,    281-82;    soil,    25, 

280,  282 
watering  operation,  27,  283-84 
See  also  Chemical  treatment  of  soil;   Con- 
tainers, treatment  of;  Dry  source  of  heat 

for  soil   treatment;    Hot-water   treatment 

of  soil;  and  Steam  treatment  of  soil 
Medicago,  see  Bur  clover  and  Alfalfa 
Meloidogyne,  see  Root-knot  nematode 
Mercuric     chloride,     treatment     of     planting 

stock,  20,  230,  232 
Methyl  bromide,  143,  200,  201-3,  206,  208-9, 
264,  267,  269,  272-73 
application,  16,  18,  124,  198,  201-2,  204 
cost,  18,  124,  137,  197-98 
effectiveness,  determination  of,  124;  in  crop 

refuse,  124,  203;  in  soil,  16,  124 
ineffective   against    Verticillium,   16,    18,  22, 

124,  203 
residual  toxicity  to  plants,  16,  124,  199 
sources,  304 
steam-methyl    bromide    for    soil    treatment, 

185 
temperatures  for  use,  124,  201-2 
toxicity  to  crops,  10,  16,  17,  18,  124,  199,  208 


treatment,  of  containers,  19,  212;  of  farm 
machinery,  201,  212;  of  soil  in  stacked 
containers,  18,  201-2,  272 

use  in  "pinto  tag"  certification,  48,  129,  201 
Micronutrients,    106-7;   supply    in    U.   C.-type 

mixes,   12,  89,  101,   106-7,  109,   110 
Microorganisms,  soil    (Section    14),   237-54 

abundance,  237-38 

antagonistic,  4,  20,  21,  24-25,  35,  250-54 

balanced  population,  21,  25,  238-40,  250-54 

beneficial,  238-40 

buffering  capacity,  299 

carbon  cycle,  246-47 

cause  of  disease,  3,  4,  33-35,  138 

competition,  21,  24,  238-40,  242,  300 

concentration  in  rhizosphere,  240-41;  in 
surface  layer,   120,  238,  240 

controlled  colonization,  see  Controlled  colo- 
nization 

crop-antagonistic,  22,  238-40 

decomposition  of  organic  matter,  89,  95, 
115-19,237,240-44 

dependence  on  green  plants,  241 

depth  in  soil,  6,  120,  238,  240 

distribution,  240 

dynamic  equilibrium,  21,  25,  238-40,  250- 
54 

effect  of  fungicide  dosage  on,  249-50;  of 
organic  matter  on,  25,  238,  240,  242-44; 
of  oxygen  on,  121,  240,  244,  298;  of  soil 
moisture  on,  21,  25,  49-50,  240;  of  pH  on, 
120-21,  245-48;  of  soil  treatment  on,  19, 
113-15,  115-19,  120,  204,  205,  248-50;  of 
soil  temperature  on,  21,  25,  240,  244 

fermentation,  138 

harmful,  238-40 

having  the  same  name,  23,  255-62 

included  in  this  manual,  34 

injurious  to  plants,  238-40 

inoculum  potential,  5,  42,  204,  260,  300 

nitrogen  cycle,  245-46 

nutrient  requirements,  242-44 

release  nutrients  in  soil,  89,  237,  244-48 

retardants  to  pathogens,  4,  20,  21,  24-25,  35, 
250-54 

spread,  see  Spread  of  microorganisms 

survival  in  soil,  21,  238,  243,  261 

types  1  and  2  in  nitrogen  conversion,  245, 
254 

variability,  255-59 
Minerals  supplied  by  soil,  10,  89,  138 
Mix,  soil,  see  Soil  mixes  and  U.  C.-type  soil 

mixes 
Mixed  infections,  261 

Mixes,  fertilizer,  see  Fertilizer  mixes  and  Fer- 
tilizers 
Mixing,  U.  C.-type  soil,  83-84,  97,  98,  105.  133, 

276-79;  uneven,  effect  on  steaming,  134 
Mobile  bin  for  soil  steaming  (type  2),  16,  132, 

148,  164,  166,  168,  193,  273,  280-81,  303 


[319] 


Moisture 
soil,  29,  87-88,  120,  125,  300;  deficiency,  ac- 
cumulative effect  of,  87-88;  relation  to 
damping-off,  5,  49-50;  relation  to  salin- 
ity measurement,  61;  see  also  Soil,  mois- 
ture 
supplied  by  soil,  87 

Molybdenum  required  by  plants,  89,  107 

Mono-ammonium  phospbate,  14,  78 

Monocalcium  pbosphate,  70,  106 

Morning-glory,  Rhizoctonia  on,  257  (fig.  124) 

Mosaic,  anemone,  21,  233;  carnation,  234;  cel- 
ery, 265;  freesia,  21,  233;  geranium,  234;  po- 
tato, 221;  ranunculus,  21,  233;  rose,  7,  21, 
51,  232,  236;  stock,  235;  tobacco,  140 

Mosses,  211 

Mother  block  (nucleus  block)  propagation, 
21,  31,  222,  235-36;  maintaining  horticul- 
tural quality,  235 

Movable  Thomas  method  for  ground  beds 
(type  18),  180 

Moving  rake  method  of  soil  steaming  (type 
23),  131,  148,  165,  166,  178,  182-84 

Multiplicity  of  nursery  soil  mixes,  disadvan- 
tages, 89-90,  93,  264-65 

Multipurpose  tank  (type  7),  16,  131,  132,  148, 
164,  166,  171-73,  176;  for  hot-water  treat- 
ment of  stock,  171-73;  for  removing  salts 
from  containers,  171-73;  for  soil  and  con- 
tainer treatment,   164,   166,   171-73 

Muriate  of  potash  (potassium  chloride),  14, 
76,  77,  78,  106,  305 

Mushroom  growers,  90 

Mustard,  host  of  stock  mosaic,  235;  seed  heat 
resistant,  139 

Mycelium,  38,  39,  44-45,  300 

Myrothecium  as  a  retardant,  25;  effect  of  ad- 
ding cellulose  to  soil  on,  251,  253;  inhibits 
Rhizoctonia,  25,  251,  253;  may  stunt  plants, 
251,  253 

Xabarn  (Dithane  D-14),  19,  207,  209;  sources, 
304 

Narcissus,  stem  and  bulb  nematode  on,  258 

Nasturtium,  215;  presoaking  seed  before  heat 
treatment,  224;  spotted  wilt,  235 

Necrosis  of  plant,  caused  by  soil  toxicity,  9, 
91-95;  bv  damping-off,  35-37,  44-45;  by  sa- 
linity, 7-9,  55-56,  58-60 

Nemagon,  206,  305 

Nematorides,  18,  21,  198,  206,  208;  chloropk- 
rin,  18,  208;  I)D  mixture,  18,  206,  209,  304; 
ethylene  dibromide  (EDB),  18,  198,  199, 
206,  209,  804;  may  increase  losses  from 
fungi,  261;  methyl  bromide,  18,  208-9;  Ne- 
magon, 206,  305;  sodium  selenate  (P-  10). 
232,  305;  Vapam,  18,  204-5,  208;  V-C  13, 
206,  805 

Nematodes,  55,  200,  206,  238,  800 
control,  15,  L8,  107,  I  OH,  202-3,  203-5 
daggei  (Xiphim  ma),  229 


foliar  (Aphelenchoides),  20,  47,  139,  220, 
232-33 

fungi  which  trap,  240 

longevity  in  soil,  261 

meadow  (Pratylenchns),  47,  139,  227,  229 

potato  root  nematode  (Heterodera),  139 

root  knot  (Meloidogyne),  see  Root-knot  ne- 
matode 

root  lesion  (Pratylenchns),  47,  139,  227,  229 

spiral  (Heiichotylenchns),  229 

stem  and  bulb  (Ditylenchus),  47,  139,  227, 
258 

sting  (Belonolaiynus),  261 

survival  in  soil,  261 

symptoms,  47 

variability,  258-59 
Nemesia,  215 
Nephthytis,    hardening    for    heat    treatment, 

223-24;  Rhizoctonia  on,  220-21;   U.  C.  sys- 
tem for,  267;   water-mold  root  rot,  220-21 
Nerium  (oleander),  220 
New  Improved  Ceresan,  treatment  of  planting 

stock,  20,  230 
New  York  soil  heater  (type  14),  142,  165,  166, 

176-77,  179,  195,  303 
Nicotiana  (tobacco)  82,  140,  214,  215,  248,  258, 

261 
Nitrate  nitrogen,   13,  95,   105,   109,   111,   115, 

116-19,    122,  243,  245,  253-54;   leachability 

from  soil,  109;  when  to  use  as  fertilizer,  14, 

105 
Nitrifiers,  see  Nitrifying  bacteria 
Nitrifying  activity  in  soil 

effect  of  soil  depth  on,  114-15,  238,  240;  of 
low  organic  nitrogen  on,  116-18;  of  pH 
on,  13,  120-21,  245-46;  of  temperature  on, 
13,  115-19,  121,  245-46 

elimination  by  steaming,  13,  95,  113-15, 
115-19,  120,  245-46,  254 

reinoculation    of    soil    by    bacteria,    13,   25, 
113-15,    116,    253-54;    without    ammoni- 
fiers,  13 
Nitrifying  bacteria,   13,   119,  239-40,  245;  ef- 
fect  of   pH   on,    13,    120,  245-46;   effect   of 

temperature  on,   13,   115,   119,   121,  245-46; 

inoculation   in   treated  soil,   13,  25,   113-15, 

116;    sensitivity    to   soil    treatment,    13,    95, 

113-15,  115-19,  120,  245-46,  254 
Nitrite,  toxicity  from,  in  soil,   121,  245 
Nitrogen    (Section    7),    108-22;   also    105,    108, 

110,  237 

content  in  organisms,  242-44 

conversion,  13,  245-46 

cycle,  245-46;  in  soil,  109,  245-46;  in  plant, 
109 

deficiency,  1 8 

essential  to  plants,  89,  106 

fixed  from  air,  89,  244-45 
fixing  bacteria,  2 15 

loss  in  organic  matter  decomposition,  65-67, 
99,  100-1,  105-7 


[  °>20  1 


relation  to  damping-off,  42-43 

starter  solutions,  13,  14,  111-12,  254 

tied  up  in  soil  by  organic  matter,  242-44; 
by  sugar,  243,  244 

See  also  Ammonium;  Fertilizer  ingredients; 
Fertilizers;  Nitrate  nitrogen;  and  Organic 
nitrogen 
Nitrobacter,  245 
Nitrosomonas,  245 
Nucleus    block    (mother   block),   21,    31,   222, 

235-36 
Nursery    industry    in    California    (Section    2), 

28-33 

amount  of  soil  used,  3,  29 

climatic  relations,  29-30,  233 

decreasing  returns,  32,  196 

distance  from  market,  31-32 

expanding  local  market,  31-32 

future  developments,  27,  193,  254 

kinds  of  plants  grown,  29 

labor  cost  increasing,  32,  96,  196,  271 

location  in  state,  28,  29 

mechanization,  see  Mechanization  in  grow- 
ing 

moving  to  rural  areas,  31-32,  274 

number  of  units,  28 

population  pressure  increasing,  31-32,  271, 
274 

production  cost  increasing,  31-32 

real-estate  development,  31,  271 

rising  land  values,  31 

size,  28-29,  33 

smog  injury,  31,  32,  274 

tax  rates  increasing,  31,  271,  274 

unit  containers  for  marketing,  25,  31,  127 

year-round  growing,  3,  7,  30-31,  49 

zoning  restrictions,  31 
Nursery  Sanitation  Code,  22-23 
Nutrients  for  plants,  availability  affected  by 

microorganisms,  89,  237,  244-48;  deficiency, 

accumulative  effect,  88-89;  excess  may  kill 

plants,  53,  54,  56-57,  64-65,  88-89 
Nutrition  research,  only  on  healthy  plants,  51 

Oak  leaves,  100 

Oak-root    fungus    (Armillaria    root    rot),    135, 
198,  220,  260-61 

Objectives  of  manual,  4,  29,  33;   microorgan- 
isms included,  34 

Obtaining  pathogen-free  planting  stock,  219- 
33 

aging  of  seed,  21,  232 

aseptic  culturing  of  growing  point,  20,  221 
chemical  treatment  of  stock,  20,  41-42,  230, 

232,  236 
continued  roguing  of  stock,  21,  232,  300 
cultured-cutting  technique,  20,  221-22,  236 
environmental  control,  220-21 
few  healthy  plants,  20,  219,  236 
grow  up  away  from  soil,  20,  219-21,  236 


heat     treatment    of    stock,    see     Hot-water 

treatment  of  planting  material 
indexing  for  viruses,  232-33 
new  seedlings,  233,  231,  236 
sanitary  practices,  see  Sanitary  practices 
select  growing  areas,  21,  233,  236 
specialist  propagator,  20,  31,  219,  233 
tip  cuttings,  20,  219-21 
use  of  true  seed,  21,  233,  234 
Ocimum  (sweet  basil),  205 
Odors  from  compost  piles,   12,  90,  271;   from 

organic  nitrogen,  122 
Ohio  soil  heater  (type  15),  165,  166,  177,  179, 

195 
Oil,  fuel  for  boilers,  16,  136,  162,  163,  164-65, 

174,  176,  185,  190,  192,  194 
Oil-drum,  method  of  soil  steaming  (type   12), 

131,  164,  176 
Oleander,  stem  and  leaf  gall,  220 
Onion,  stem  and  bulb  nematode  on,  258 
Oospores,   function   in   fungus   carryover,   44- 
45;  occurrence,  44;   useful   in  disease  diag- 
nosis, 44 
Organic  matter,  11,  104,  109,  120 
carbon/nitrogen  ratios,  242-44 
decomposed  by  microorganisms,  89,  95,  115— 

19,  237,  240-44 
effect  on   aeration,  99;   on   microorganisms, 
25,  238,  240,  242-44;  on  soil  salinity,  15, 
65-67 
high-nitrogen  materials,  243-44 
low-nitrogen  materials,  242-43 
particle  size,   relation   to   soil   leaching,    15, 

65-67 
rate  of  decomposition,  241-42 
relation   to   aeration,   98-99,    120,   143,   149, 

240;    to  steam  penetration,   127-28 
role  in  soil  toxicitv,  95 
shrinkage,  12,  85,  89-90,  100-1,  241 
steps  in  decomposition,  241,  246-47 
Organic  nitrogen,  105,  108-9,  245 
application  rate,  13,  71-75 
applied  as  top  dressing,  13-14,  118-19,  121, 

122 
conversion  in  soil,  13,  53,  109,  112,  113,  245 
effect  of  aeration  on,  121;  of  chemical  treat- 
ment on,   119;  of  microorganism  popula- 
tion on,    108,   113,   120,   122;   of   moisture 
on,   121;  of  quantity  applied,  on  conver- 
sion of,  117,  120;  of  steaming  on,  94,  108- 
9,  113-15,  115-19;  of  temperature  on  con- 
version of,  13,  53,  105,  112,  115-19,  120 
insoluble  in  water,  108 
minimal   nitrogen    level    supplied,    94,    121, 

122 
relation   to   ammonium    injury,    13,    14,  95, 

112,  113;  to  salinity,  70,  76' 
relative  rates  of  conversion.  13.  14.  118,  121 
soil  organisms  converting,   13.   105,  245 
unavailable  to  plants,  13-14,  108 


[321] 


Origin  of  plant,  relation  to  distribution,  86, 
91,  109 

Orobanche,  261 

Orthocide  406  (captan),  19,  43,  207,  209,  304 

Osmotic  concentration  of  soil  solution,  95,  300 

Overhead  watering,  27;  relation  to  disease 
spread,  20,  23,  38,  39,  45;  relation  to  salin- 
ity burn,  9,  15,  53-54,  64 

Oyulinia  azaleae,  21,  30,  232,  233,  234 

Owgen,  87,  121,  244,  298 

Oystershell  lime,  106 

P-40  (sodium  selenate),  232,  305 

Package  boilers,  see  Steam-generating  equip- 
ment 

Pallet  for  stacking  flats,  22,  25,  26,  166,  171, 
174,  211,  212,  272-73,  276-77,  279,  280-81 

Palms,  29,  284 

Panics  in  nursery  business,  7,  51 

Pansy,  42,  43,  111,  207,  215,  264,  269 

Papaver  (poppy),  111,  215,  233 

Paraformaldehyde,  19,  212-13 

Parasite,  299,  300 

Parasitic  organisms  in  soil,  attacking  patho- 
gens, 24,  238-40,  300;  competition  escape  by 
infecting  plant,  21,  238 

Parasitism,  specialization,  5,  21,  46-47 

Parathion,  sources,  305;  treatment  of  plant- 
ing stock,  20,  232 

Particle  size,  see  Soil,  compaction,  and  Soil, 
drainage 

Parzate  (zineb),  267,  305 

Pathogen,  127;  definition,  300;  depth  in  soil, 
6,  120,  259;  rate  of  increase,  218;  soil-in- 
habiting, 20,  218-19;  sources,  123,  217;  vari- 
ability, 255-59 

Pathogen-free  planting  stock  (Section  13), 
217-36;  also  frontispiece,  1,  6,  11,  19-20,  22, 
24,  31,  34,  48,  111,  123;  benefits  from  use, 
49-51,  217,  218;  importance,  7,  29,  31,  43, 
217;  maintaining,  21,  31,  226,  234-36; 
methods  of  obtaining,  20,  21,  31,  41-42,  219- 
33,  236,  300;  obligation  of  nursery  to  pro- 
duce, 6,  36,  262;  report  diseased  stock  to 
propagator,  20,  233 

PCNB  ( 1  erraclor),  19,  43,  205,  207,  208-9,  305 

Pea,  43,  215;   Fusarium  wilt,  257 

Peach,  root-knot  nematode  on,  259 

Peanut,  root-knot  nematode  on,  259 

Peat,  black  (sedge),  69,  98,  265;  source  of  sa- 
linity, 15,  53,  98,  101-5;  of  soil  toxicity,  95 

Peat,  hypnum,  69,  104 

Peat,  sphagnum,  12,  81,  91,  94,  97,  98,  99,  101, 
104,  115-19,  112,  144,  228,  242,  268,  275,  276 
buffering  capacity,  L06 
cost,  69,  85,  100 

cllcd  on  teachability  of  soil,  65-67 
formation,  244 
heal  conductivity,  2!)!) 

ingredient  <>f   l  .  C.  type  soil  mixes,  72-75, 
77,  78,81,  83-84,  85,93,  211 


micronutrients,  107 

mixing,  83-84 

nitrogen  content,  116,  244 

pH,  69,  106,  120 

seed  cover  for  suppressing  damping-off,  43 

types,  69,  104 

water  retention,  69,  100 

weight,  69 

Pelargonium  (geranium),  20,  21,  29,  32,  35, 
215-16,221,234,261 

Pelleting  of  seed,  25,  283 

Pellionia,  hardening  for  heat  treatment,  223- 
24;  hot-water  treatment,  229;  Rhizoctonia 
on,  220-21;  U.  C.  system  for,  267;  water- 
mold  root  rot,  220-21 

Penicillin,  255-56;  as  a  retardant,  25,  251-52; 
effect  of  pH  on,  251 

Peperomia,  Rhizoctonia  on,  220;  U.  C.  system 
for,  267;  water-mold  root  rot,  220 

Pepper,  25-26,  29,  111,  214-15,  251-54,  256-57, 
260,  264-65;  heat  treatment  of  seed,  226; 
Phytophthora  root  rot,  258;  Rhizoctonia  in 
seed,  25,  41-42,  43,  226,  260;  root-knot  nema- 
tode on,  259;  spotted  wilt,  235 

Perforated  pipe  method  of  soil  steaming 
(type  20),  16,  128,  131,  132,  138,  148,  150, 
152,  156,  165,  166,  178,  180-83,  184,  194 

Perlite,  12,  97,  100,  101,  227 

Permeability  of  soil,  64—65;  affected  by  clay 
content,  143 

Peronospora,  46-47 

Persea  (avocado),  45,  258 

Persica  (peach),  259 

Petunia,  25,  207,  215,  264,  283;  ammonium 
injury,  13,  111,  112,  113;  chlorosis,  113; 
formaldehyde  injury,  213;  Rhizoctonia  on, 
43;  Sclerotinia  on,  46 

pH,  95,  299,  300 

desirable  ranges  for  plants,  106 
effect  of  peat  on,  69,  106,  120 
effect   on    ammonifiers,    120,    246;    on    con- 
trolled   colonization,    251;    on    nitrifying 
activities,    13,    120-21,   245-46;    on    Peni- 
cillium,  251;  on  soil  microorganisms,  120- 
21,  245-48;  on  Streptomyces,  251;  on  Tri- 
chodcrma,  251 
of  U.  C.-type  soil  mixes,  69,  70 
relation  to  nutrient  availability,  247 

Phaseolus,  see  Bean  and  Lima  bean 

Philodendron,  bacterial  stem  rot,  220,  229; 
heat  treatment  of  plants,  229,  231;  Rhi- 
zoctonia on,  220-21,  229;  root-knot  nema- 
tode on;  259;  II.  C.  system  for,  267;  water- 
mold  root   rot,  220-21 

Phlox,  111,  113,  261 

Phomopsis,  233 

Phosphorus  and  phosphate,  12-13,  70,  95, 
106,  108,  110,  115;  essential  to  plants,  89; 
fixation,  77,  78;  loss  in  organic  matter  de- 
composition, 100;  mono-ammonium  phos- 
phate, 11,  78;  monocalcium  phosphate,  70, 


[322] 


106;     rendered     available     bv     microorgan- 
isms, 237,  247 
Photosynthesis.  247 
Phvcomvcetes.  301 
Phyllosticta,  233 
Phytophthora,   5,    7,    37,   44-46,   49,   82.   220: 

capsici.  258;  cinnamomi,  45-46,  218-19.  258; 

parasitica  f.  nicotianae,  258,  261;  richardiae. 

229;  variability  in  pathogenicity,  258 
Pice  a  (spruce),  258 
Pike's  Peak  plastic  covering.  303 
Pine.    Pythium    root    rot,    258;    Phytophthora 

root  rot.  46;  shavings,  65-67,  244 
"Pinto  tag"  in  certification.  48.  129.  201 
Pipe  spacing  in   relation   to  steam   flow   rate. 

130-31,  153,  180,  182 
Pipe,   steam-distributing.   195-96 
Pisum  <pea).  43.  215.  2"7 
Plant,  composition.  13s 

Plant  intioduction.  ancient  example.  91-93 
Planter    boxes.    131:    fertilizer    schedule.    81; 

soil  mixes.  81,  84 
Planting   depth,    relation    to    damping-off,    5. 

43 
Planting  equipment.  25-27.  264,  283,  302 
Plastic  containers.  53.  211.  212 
Plicaria  as  a  retardant.  25 
Plow,  steam.  182 
Poinsettia.   29:    Rhitoctonia   on.  257;    Thiela- 

viopsis  on.  258:  U.  C.  svstem  for,  266 
Polvethvlene.   212.   213,   216,   221;    sheets    for 

covering   soil    chemically    treated,   201:    for 

steaming  soil,  170.  178-80,  275.  303;  under 

flats  on  ground,  23,  41 
Polyvinyl  sheets,  201 
Poppv.  111.  215;  bacterial  leaf  spot,  233 
Population  pressure  on  nurseries,  31-32.  271, 

274 
Porosity  of  soil,  see  Soil  porosity 
Porous  canvas  hose,  303 
Post-emergence    damping-off,    5,    35-36,    249, 

257 
Pot  filler.  25.  168,  272-73.  280,  282,  302 
Pot   plants.    169:    fertilizer   schedule.   80,    119: 

soil  mixes  for.  12.  69.  73.  77.  79-80,  266-67: 

U.  C.  svstem  for.  266-67 
Potassium  and  potash.  95.  99.   106,   108,   109, 

110.  115:  availability  altered  bv  microorgan- 
isms.   237.   247:   contributes    to   salinity,    13; 
essential  to  plant.  89,  106;  frit.  76.  106;  loss 
in     organic     matter     decomposition.      1 
methods  for  mixing  in  soil.  70.  83-84,  105 

Potassium  chloride  14.  76.  77.  7V.  106.  305 
Potassium  nitrate.  13.  70-75,  94.  105,  106.  110, 

111.  119.  177;  mixing.  70,  105 

Potassium  sulfate  14,  53,  70-75,  76,  77,  91.  94. 

106,   111,   119 
Potato,    Rhizoctonia   on.    257;    ring   rot,    216: 

scab,  247;  virus  X  on.  221 
Potato    root    nematode    (Heterodera).    lethal 

temperatures.   139 


Pothos,  root-knot  nematode  on,  259 

Pots.  126.  127.  150.  196.  210,  254,  272-73;  salt 
accumulation.  9.  15,  19.  53,  56,  211-12;  treat- 
ment. 15,  40,  162,  170,  171,  173,  201,  204, 
211-16 

Potting-on.  soil  mixes  for.  71.  72.  73,  75,  119 

Pounds  of  steam,  300 

Pratxlenchus  (lesion  and  meadow  nemato< 
47.  139.  227,  229 

Preemergence  damping-off,  5,  35-36,  249,  257 

Preparation 
of  plant  material  for  hot-water  treatment, 

20.  224 
of   U.   C.-tvpe   mixes,   hand   mixing.   83-84, 
8;   machine  mixing,  83-84,  98,  276-79 

Pressure  cooker,  see  Autoclave 

Pressureless  steam,  see  Free-flowing  steam 

Pressure  steam  for  soil  treatment.  157,  158, 
164-65,  167-74.  178-84,  301;  advantages.  129. 
158;  disadvantages.  15-16.  129,  135;  heat 
content.  129,  157-58;  pressureless  in  ^oil. 
129.  149:  thermal  efficiency.  129.  157-58;  see 
also  Autoclave  and  Equipment  for  soil 
heating 

Prevention,  emphasized  in  plant  disease.  6.  10, 
48 

Primula  (primrose),  soil  mixes  for.  79;  Thie- 
laviopsis  on.  258 

Production  block  (increase  block  .  222.  235-36 

Production  cost,  reduction  needed.  31-32 

Propagating  material,  29;  pathogen-free,  see 
Maintenance  of  pathogen-tree  stock  and 
Obtaining  pathogen-free  planting  stock: 
spread  of  pathogens.  6,  19-20.  29,  217: 
treatment,  chemical,  20,  42.  230.  232;  treat- 
ment, heat.  20.  164-65.  223-30,  236 

Propagation  operations,  segregation  from 
commercial   production,   21,   22-23 

Propane,  fuel  for  boilers.  162.  192.  195 

Proportioners.  for  liquid  fertilizer  application. 
283,  302:  for  Yapam  application.  204 

Protective  seed  treatment.  20-21.  42.  230,  232: 
does  not  eradicate  pathogens.  230 

Protein.  241 

Protozoa  in  soil.  21,238 

P     udomonas.  47 

Pseudotsuga  | Douglas  Br),  46.  65-67 

Psorosis,  citrus.  232 

Puratized  Agricultural  Spray,  228.  305 

Purex  isodium  hypochlorite.  216.  221-22. 
226.  305 

Pythium,  5.  35,  37.  44-46.  205,  226-27:  lethal 
temperatures.  139;  variability  in  parasitism. 
258 


Quantity    of   pathogens.    300;    importance 
42,  204.  260 

Quantity  of  steam,  calculation.  132-33:  rela- 
tion to  soil  volume  and  time.  132-33.  149- 
52;   required   for  soil   treatment.   16.  159-60 


[  323  ] 


Rack  for  steaming  pots,  280,  282 

Radiation  of  heat,  149,  159;  definition,  141; 
factors  affecting  in  soil,  142;  importance  in 
soil  heating,  144,  150,  155;  relation  to  pore 
size,   144 

Rainfall,  free  of  salts,  15;  relation  to  plant 
distribution,   86 

Rake  method  of  soil  steaming  (type  21),  135, 
148,  150,  165,  166,  178,  181,  183 

Ranunculus,  mosaic,  21,  233 

Raphanus  (wild   radish),  235 

Raspberry,  crown   gall,  258 

Real-estate  development,  effect  on  nurseries, 
31,271 

Recontamination  problem,  11,  20,  24,  93,  96; 
balance  in  soil  organisms,  21,  25,  238-40, 
250-54;  danger  from  inadequate  soil  treat- 
ment, 22,  249-50;  disease  greatest  from  con- 
tamination of  treated  soil,  20,  22,  249;  effect 
on  organic  nitrogen  breakdown,  113-15, 
120,  122;  luxuriation  of  first  contaminant, 
22,  248-49;  prevented  by  sanitation,  22-23, 
250 

Red  stele  of  strawberry,  220 

Redwood,  used  in  soil  mixes,  fiber  (bark),  98; 
sawdust  and  shavings,  69,  81,  244 

References,  by  section,  285-98 

Relative  humidity,  11,  15,  29-30,  42-43,  146- 
47 

Reliability  of  soil  mixes,  68 

Research  use  of  U.  C.-type  mixes,  81-83; 
makes  possible  the  selection  of  uniform 
plants,  83 

Respiration,  87,  246-47 

Retardant  microorganisms,  4,  20,  21,  24-25, 
27,  35,  113-115,  116,250-53,  254,  300 

Returns,  decreased,  32,  196;  offset  by  reduced 
competition,  32;  offset  by  reduced  produc- 
tion cost,  32 

Rhizoctonia  diseases,  35,  38;  see  also  Damp- 
ing-off 

Rhizoctonia  solani,  5,  35,  37,  45,  46,  198,  214, 
218,  220-21,  226-29,  231,  233,  239-40,  249- 
50,  251-53,  266 
aerial  types,  256,  259 
air-borne  spores  absent,  38 
controlled  by  retardants,  25,  205,  207,  208 
depth  found  in  soil,  259 
fungicides  and  inoculum  potential,  260 
increasing  importance,  37,  12 
lethal  temperatures,  139 
longevity  in  soil,  201 
recognition  in  field,  38,  39 
relation   to  Fusarium  wilt  of  aster,  49-50; 
to  soil  carbon  dioxide,  259;  to  soil  mois- 
ture, 49-50;   to  temperature,  43,  259 
saprophytic  l<»i  ms,  1 1,  257 
seed  i  ransmission,  40-42 
spread,  38-42 
structure,  38,  39,  II 
subtei  ranean  i\  pes,  259 


survival  in  soil,  38,  261 

variation  in  host  range  and  virulence,  38, 

44,  256-57 
Rhizoctonia  "story,"  38-44 
Rhizomorph,  261 

Rhizosphere,  240-41;  definition,  240 
Rhododendron,  salinity  injury,  58 
Ribbon  mixers,  see  Concrete  mixers 
Rice  hulls,  12,  69,  80,  81,  97,  98,  267;  potassium 

content,  99-100,  267 
Rock,  99 

Roguing  of  planting  stock,  21,  232,  300 
Rollers,  steel,  26,  272-75,  277,  279,  282 
"Root  action,"  51,  138 

Root  distribution  in  U.  C.-type  mixes,  82-83 
Root  divisions  unsafe  for  propagation,  22,  41- 

42 
Root  functions,  51,  87,  89 

Root  injury,  effect  on  plant  responses  to  cul- 
tural   practices,   51;    from    ammonium,    13: 

from  drying  soil,  268;  from  salinity,  8-9,  58- 

60;  from  soil  toxicity,  9,  113 
Root-knot  nematode  (Meloidogyne),  47,  226- 

27,  229,  233,  239,  261;  lethal  temperatures, 

139;    number   of  eggs   laid,   218;   obtaining 

clean  stock,  220;  variability  in  pathogenicity, 

258-59 
Root  lesion  nematode  (Pratylenchus),  47,  139, 

227,  229 
Root  nodules  on  legumes,  245 
Root  rot,  34,  36-37,  202;  Armillaria,  135,  198, 

220,   260-61;    Fusarium,   229,   256;    Rhizoc- 
tonia,   5,    256;     Thielaviopsis,    258;    water 

mold,  see  Water-mold  root  rots 
Root  secretions,  259 

Root  tips,  loss  of,  importance  to  plant,  51 
Rose,  29,  35,  126,  268 

chlorosis,  107 

cultured-cutting  technique  with,  20,  221 

dagger  nematode  on,  229 

heat  treatment  of  plants,  229 

mosaic,  7,  21,  51,232,236 

Rhizoctonia  root  rot,  43 

root-knot  nematode  on,  229,  233 

root-lesion  nematode  on,  229 

spiral  nematode  on,  229 
U.  C.  system  for,  268 

Verticillium  wilt,  221,256 
Rotating   drum    soil    heaters,   external    flame, 

batch  type  (type  35),  165,  191,  303;  external 

flame,  continuous  type  (type  31),  165,  190, 

273;  internal  flame  (type  30),  165,  185,  187, 

189-90,  191,  303 
Rotating  drum  soil  heaters,  knife  injector  (type 

28),  148,  165,  166.  185-87,  273;  electric  (type 

33),  165,  189-90,  195 
Rotating    screw    soil    healers,    external    flame 

(type  31),  165,  190;  steam  (type  29),  10,  165, 

187-88,272.  303 
Rubberized  (loll)  Eoi  soil  treatments.  178,  180, 

182,  L83 


[  324  ] 


Rubus  (raspberry  and  loganberry),  258 

Ruckl  method  for  soil  steaming  (type  1),  132, 

148,  164,  166,  167-68,  180 
"Running  out,"  235 

Rust,  snapdragon,  incubation  period,  231;  in- 
troduction   to   new   areas,   218;    number   of 

spores  produced,  218 
Saintpaulia,  foliar  nematode  on,  232;  salinity 

injury  to,  9,  53,  119 
Salinity   (Section   4),   52-67;   also   frontispiece, 

1,  11,264,300 

accumulation,  in  clay  pots,  9,  15,  19,  53,  56, 
211-12;  in  plant,  9,  55-56,  64;  in  soil  sur- 
face, 9,  55-56 

dangerous  level,  15,  61,  63,  65 

definition,  52 

deposit  on  leaves,  9,  63-64 

differences  in  plant  susceptibility,  7,  55,  61 

injury  to  plants,  see  Salinity  injury,  below 

leachate  not  good  index  of  salts  in  soil,  65- 
67 

leaching  soil  to  prevent,  9,  11,  14-15,  53-54, 
57,  63,  65-67;  effectiveness  related  to  wa- 
ter salinity,  9,  14,  63 

measurement,  9,  14,  60-63,  84,  300 

necessary  to  use  excess  water,  9,  14,  63 

reduces  available  water,  87 

relation  to  fertilizer  application,  9,  15,  30, 
52,  53,  57,  64-65,  70,  106,  119-20 

role  in  soil  toxicity,  9,  95,  98 

safe  levels,  61,  63,  65 

selection  of  land  to  prevent,  64 

soaking  clay  pots,  19,  53,  171-73,  211-12 

sodium  in  water,  64 

sources,  9,  30,  53-54,  70,  116,  275 

use  deionized  water,  15,  57,  63-64 

water  quality,  related  to,  9,  11,  14,  30,  63- 
64;  characteristics,  63 
Salinity  injury,   1,  7-9,  55-56,  58-60,  87,  93, 

121,265 

aggravates  Botrytis  diseases,  8,  9,  46;  damp- 
ing-off,  5,  7,  9,  33,  42,  49-50,  55,  266 

effect  of  bright  sunlight  on,  15,  55;  of  hu- 
midity on,  15,  55;  of  organic  matter  on, 
15,  65-67;  of  soil  drainage  on,  15,  53-54, 
57,  64;  of  soil  moisture  on,  9,  11,  15,  33, 
49,  53,  55-57,  64 

from  leaf  absorption,  9,  15,  53-54,  64 

from  overhead  sprinkling,  15,  53-54 

humidifying  to  reduce,  55,  57,  64 

reduced  by  U.  C.-type  soil  mixes,  10,  15,  53, 
64,  65-67,  90,  211,  265-66,  270 

relation  to  plant  distribution,  86 

shading  to  reduce,  55,  57,  64 

variable  effects,  55,  58 
Salts,  see  Salinity 
Sand,  coarse,  142,  144 

aeration,  98,  143 

determination  in  fine  sand,  103-4 

permeability,  effect  of  clay  on,  143 

porosity,  143 


retention  of  minerals,  99 

seed  cover  for  suppressing  damping-oft,  4:> 

specifications,  102-3 

unsuitability  for  U.  C.-type  mixes,  68,  97 

use,  in  J.  I.  composts,  91;  in  propagation, 

126,  146,  184;  in  sand  culture,  68 
Sand,  fine,  for  U.  C.-type  soil  mixes,  12,  71-71, 
77,  83-84,  93,  97,  101,  110,  264,  268 
aeration,  98 
availability,  12,  275 
bacteria  sparse  in  deep  source,   13,  113-15, 

240 
buffering  capacity,  106 
content  of  coarse  sand  and  of  silt  and  clay, 

12, 103 
cost,  85,  100 

determination  of  particle  size,  103-4 
gravel    and    sand   quarries,   source,    85,   97, 

103-4 
porosity,  143 
retention  of  minerals,  99 
sources,  85,  97,  103-4,  275 
specifications,  12,  69,  101-4 
unique  ingredient  of  mix,  68 
Sand  culture,  68 
"Sand  snake,"  explained,  202 
Sanitary  practices,  frontispiece,   1,  6,   11,  22- 
23,  111,  123,  129,236,250 
containers  on  ground,  6,  22,  23,  24,  41 
covering  containers,  6,  23,  41 
degree  of  cleanliness  required,  24 
dipping  cuttings  in  water,  5,  22,  38 
discard  flats  with  diseased  seedlings,  22 
glasshouse  cleanup  program,  213,  215 
isolation  house,  21,  23,  24,  234-35,  261 
keep  hose  nozzle  off  ground,  5,  23,  24,  38-40 
microorganisms  of  the  same  name,  23,  255- 

62 
overhead  watering,  20,  23,  27,  38,  39,  45 
pathogen-free      stock,      see      Pathogen-free 

planting  stock 
plant  clean  stock  only  in  treated  soil,  22,  48 
removal  of  diseased  parts,  21,  232 
scattering  dust  in  handling,  23,  41 
segregate  propagation  activities,  21,  22-23; 

treated    from    untreated    containers,    22; 

nursery  operations,  21,  163,  166,  276 
spattering  soil  in  watering,  23,  24,  220 
taking  of  cuttings,  22 
treating,    containers,    23,    24;    floor    before 

dumping  soil,  18,  22,  134,  166,  199;  soil  in 

containers,  22,   126-27;   tools,   19,  22,  23, 

24,  40,  48,  201,  226 
treating  soil  in  containers,  22,  126-27 
unnecessary  handling  of  soil,  23 
use  treated  containers  for  treated  soil,  22. 

210,278 
walking  over  planted  flats,  6,  23,  24,  40 
washing  hands,  23 
Sanitation  Code,  22-23 
Sansevieria,  chlorosis,  107 


[  325  ] 


Saprophytes,  238-40,  300 

Saprophytic  organisms  in  soil,  238-40 

Saturated-soil  extract  method  for  salinity 
measurement,  61-63;  comparison  with  dilu- 
tion methods,  61;  compensation  for  elec- 
trode, 63;  converting  reading  to  ppm,  61; 
description,  62-63;  importance  in  salinity 
measurement,  61;  readings  on  U.  C.-type 
mixes,  65,  70,  266 

"Saucepan"  soil  steamer,  176 

Sawdust,  12,  69,  80,  81,  97,  98,  99,  100,  242-43, 
244;  cost,  85,  100;  decomposition  of,  nitro- 
gen required,  99,  242-43 

Sderotia,  38,  39,  46,  300 

Sclerotinia  camelliae,  21,  232,  233,  234 

Sclerotinia  cottony  rot,  white  blight,  5;  con- 
ditions favoring,  46;  life  history  of  causal 
fungi,  46;  number  of  spores  produced,  218; 
symptoms,  46-47 

Sclerotinia  minor,  5,  46 

Sclerotinia  sclerotiorum  5,  46;  lethal  tempera- 
tures, 139 

Sclerotium  crown  rot,  139,  226 

Sclerotium  rolfsii,  226;  lethal  temperatures, 
139 

Screening  soils,  276-79 

Screw-type  soil  treaters,  see  Rotating  screw 

Sedge  peat,  see  Peat,  black 

Seed,  infection  of  by  Rhizoctonia,  41-42 

Seed  decay,  4,  35-36 

Seed  germination,  relation  to  salinity,  9,  42, 
55;  soil  mixes  for,  74 

Seed  transmission,  6,  40-42,  217,  249,  255,  260; 
Alternaria,  229,  233;  bacteria,  229;  Fusa- 
rixim,  47;  Phytophthora,  45;  Rhizoctonia, 
41-42,  260;  Sclerotinia  sclerotiorum  and 
minor,  46;  Septoria,  226;  viruses,  7,  21,  232- 
34 

Seed  treatment 

chemical,  230;  eradicative,  230;   protective, 
20,  21,  42,  230,  232;  protective,  relation  to 
soil-inoculum  potential,  42 
heat,  see  Hot-water  treatment  of  planting 
material 

Seed  vitality  and  damping-off,  5,  43 

Seeding,  mechanical,  25-26,  264,  283,  302;  com- 
mercial use,  25,  264,  282;  seed  size  in  rela- 
tion to,  25,  283 

Semesan,  19,  207,  209;  injury  to  roses,  207,  209; 
injury  to  seedlings,  207;  sources,  305 

Senecio  (cineraria),  1 1 1 

Separator  strips  lor  stacking  Hats,  201,  279 

Septoria  late  blight  of  celery,  21,  51,  226,  232; 
number  of  spores  produced,  21 H 

Septoria  leal  spot  of  chrysanthemum,  233 

Sewage  sludge,  15 

Shading  ol  plains,  effecl  on  damping-off,  5, 
42-43;  on  salinity  injury,  15,  55 

Shalil  peach,  259 

Shavings,  wood,  HO,  HI,  97,  98,  99,  100,  242-43, 
211,  268;  (ost,   loo;  died   on   teachability, 


65-67;   require   nitrogen   in   decomposition, 
99,  242-43 

Shepherd's  purse  seed,  heat  resistant,  139 

Shredder  for  peat  moss,  98,  276 

Shrinkage  of  organic  matter  in  composting, 
12,  80,  85,  89-90,  97,  100-1,  241 

Shrubs,  29 

Silt,  97,  98-99,  104;  determination  in  fine  sand, 
103-4 

Sisalkraft  cover  for  soil  treatments,   178,  303 

Skip-load  tractor,  25,  84,  168,  272-73,  276,  278- 
79,  280 

Slugs,  lethal  temperatures,  139 

Smog,  injury,  31,  32,  274 

Snapdragon,  25,  111,  207,  215,  263;  ammonium 
injury,  13,  113,  253;  anthracnose,  233; 
chlorosis,  113;  damping-off,  47;  downy  mil- 
dew, 46,  47;  methyl  bromide  injury,  16,  18; 
Phyllosticta  leaf  spot,  233;  root-knot  nema- 
tode on,  259;  rust,  218,  234 

Snyder's  plastic  covering,  for  soil  treatment, 
303 

Sodium,  64,  106,  109,  300;  in  water,  related  to 
quality,  64,  106,  109 

Sodium  acetate,  112 

Sodium  hypochlorite,  216,  221-22,  226;  sources, 
305 

Sodium  nitrate,  105 

Sodium  oxalate  solution,  preparation,  103-4, 
305 

Sodium  selenate  (P-40),  sources,  305;  treat- 
ment of  planting  stock,  232 

Sodium  sulfate,  65-67 

Soft  rot  (bacterial),  21,  35,  226-27,  233,  235 

Soil 

absorption  of  water,  gases,  and  salts,  95 

analysis,  84,  99 

clods,  avoidance   by   U.   C.-type  mixes,  98, 

133,  161,  276;  relation  to  steaming,  15, 
127,  133,  134,  140,  149,  160-61,  190 

compaction,    relation    to    fumigant    move- 
ment, 89,  142;  to  heat  transmission,  130, 

134,  142;  to  particle  size,  97,  99,  103,  104, 
142;  to  steam  movement,  130,  134,  142, 
H8-49,  152,  155,  156 

conditioners,  15,  18,  65-67 

drainage,    relation    to    aeration,    60,   99;    to 

particle   size,   97;    to   root   rot,  44-45;    to 

salinity,   15,  53-54,  57,  64;   restriction   by 

container  boundary,  61,  87 
drenches,  see  Spot  treatment 
functions  for" plants,  10,  86-89 
grinding  of  clods,  98,  133,  161 
handling,  separating  various  operations,  21, 

163,  166,  276 
heal    capacity,    compared    with    water,    133, 

160 
heal  conductivity,  299 
moisture,  29,  87-88,  120,  125,  300;  effecl  on 

damping-off,  5,  33,  .'50,  49-50;  effect  on 

permeability,  143;  effect  on  pore  size,  1  13; 


[326] 


effect  on  soil  steaming,  15,  130,  133,  131, 

149,  155,  160,  193;  high  levels  reduce 
salinity  injury,  9,  11,  15,  33,  49,  53,  55- 
57,  64;  relation  to  salinity  measurement, 
61;  resulting  from  steaming,  135,  150-51; 
see  also  Moisture,  soil 

nonspecificity  for  crops,  10,  86 

particle  size,  in  relation  to  concretion,  99, 
103,  104,  142;  see  also  Soil,  compaction, 
and  Soil,  drainage,  above 

permeability,  64-65,  143 

porosity,  effect  of  clay  on,  99,  143;  effect  of 
moisture  on,  143,  149;  in  various  soils,  98- 
99,  143-44;  porosity  and  pore  size  com- 
pared, 143-44;  relation  to  carbon  dioxide 
diffusion,  143;  relation  to  convection 
movement  of  steam  and  fumigants,  143- 
44;  relation  to  heat  transmission,  141-44, 
149 

quantity  used  by  California  nurseries,  3,  29 

relation  to  disease  spread,  38,  45,  123,  217 

relation  to  plant  distribution,  86,  91 

source  of  mineral  nutrients,  89,  107 

source  of  salinitv,  15,  30,  53-54 

specific  heat,  301 

structure,  125 

supplies  most  of  plant  requirements,  138 

temperature,  effect  on  ammonifiers,  121, 
245-46;  on  damping-off,  5,  6,  43;  on  dis- 
ease, 50,  260;  on  organic  matter  decom- 
position, 53, 115-19 

testing  equipment,  303 

top,  cost,  85 

weight,  135 

See  also  Aeration  of  soil;  Chemical  treat- 
ment of  soil;  Dry  source  of  heat;  Equip- 
ment for  soil  handling;  Hot-water  treat- 
ment of  soil;  Mechanization  in  growing; 
Microorganisms,  soil;  Soil  mixes,  below; 
Steam  treatment  of  soil;  and  Toxicitv 
after  treatment  of  soil 
Soil-  and  refuse-borne  viruses,  127,  140 
Soil  mixes  (sections  5  and  6),  68-107 

conventional  types,  91 

Einheitserde,  94,  96 

historv  of  development,  90-94,  110 

ideal,  97,  109-10 

John  Innes  composts,  91,  93 

mix  A,  12,  69,71,  77,  78,  85 

mix  B,  12,  69,  72,  77,  78,  79,  80,  81,  85,  111, 
112,  115, 117,  135 

mix  C,  12,  69,  73,  77,  78,  79,  80,  81,  85,  149, 

150,  153,  154,  156 

mix  D,  12,  69,  74,  77,  78.  80,  81,  85 

mixE,  12,69,75,  77,  78,81,85 

multiple  vs.  single  mixes,  86.  89-90,  91 

philosophies  behind,  91,  93-94 

U.  C.  type,  69-76,  93-94,  110;  see  also  U.  C. 
type  soil  mixes 
Solanum,  see  Eggplant  and  Potato 
Soluble  salts,  see  Salinity 


Solubridge   for  salinitv    measurement,   9,   60- 

63,  299,  300,  303 
Sore-shin  damping-off  (wire-stem;,  5,  35-36,  43 
Sources,   of   equipment,   302-4;   of   fungicides 

and  chemicals,  304-6 
Soybean,  Rhizoctonia  on,  257 
Space  in  nurseries,  reduced  b\   U.  C.-type  soil 

mixes,  10,  12,  89-90,  100 
Specialist  propagator,  20,  31,  219,  233 
Specific  heat,  301;  of  soil,  135;  of  water,  135; 

of  water  vs.  soil   15,  133,  131,  160,  196,  301 
Spergon  (chloranil),  230 
Sphagnum  peat  moss,  see  Peat,  sphagnum 
Spike  method  for  soil  steaming  (type  21),  135, 

148,  150,  165,  166,  178,  181,  183 
Spiral  nematode  (Helicliotylenchus),  229 
Spore,  5,  46,  250,  298,  299,  301 
Spot    treatment,   fungicidal,    19,   43,   48,   207, 

209;  application,  19,  207;  importance  of  cor- 
rect dosage,  207,  249-50;  salvage  technique, 

19,  48,  207 
Spotted-wilt  virus,  21,  221,  235;  endemic  cen- 
ters of  infection,  235 
Sprays,  use,  4,  34 
Spread  of  microorganisms,  b\ : 

air-borne  spores,  5,  38,  46 

cloth  flat-covers,  6,  23,  41 

containers,  5,  39-40,  45,  48 

hormone  solutions,  5,  22,  38 

hose  nozzle,  5,  23,  24,  38-40 

placing  container  on  ground,  6,  22,  23,  24, 
41 

planting  material,  6,  19-20,  40-42,  43,  45-46, 
48 

seed,  6,  40-42,  45,  47,  217,  229,  233,  249,  255, 
260 

soil  particles,  5,  38,  39,  48 

tools,  6,  40-41,  45,  48,  217,  249 

water,  5,  11,38,45,48 

workers'  hands  or  feet,  6,  23,  24,  40 
Spruce,  Pythium  root  rot,  258 
Stacking  of  flats,  for  seed  germination,  264;  for 

soil  treatment,  131,  173,  189,  201-4,  211 
Starch,  241 
Started  plants,  29 

Starter  solutions  (nitrogen),  13,  14,  111-12,  254 
Steady  state  in  heat  transmission,  145-46 
Steam 

box,  for  bulk  soil  (fixed  front)  (tvpe  4a),  16, 
132,  148,  156,  164,  166,  167,  169-70,  176. 
273,  303;  for  bulk  soil  and  containers  (re- 
movable front)  (tvpe  4b),  16,  17,  131,  132, 
148,  156,  164,  166,  169-70,  303 

B.t.u.  requirements  per  cu.  ft.,   16,   159-60 

calculation  of  volume  required  for  soil  treat- 
ment, 132-33,  159-60 

definition,  141 

delivered,  calculation,  301 

differentiated  from  heat.  142 

distribution,  see  Distribution  system  for 
steam 


[327] 


Steam,  continued 

escape  from  treated  soil,  128,  130,  131,  149- 
50,  153,  155,  159-60 

flow  rate,  relation  to  pipe  spacing,  130-31, 
153,  180,  182 

forms,  for  soil  treatment,  15-16,  129-30,  135, 
149,  157-59,  164-65,  166,  196,  211,  300,  301; 
equipment  for  generating,  191-92;  for 
specific  types  of  soil-treating  equipment, 
167-74,  178-84,  187;  with  steam-air  mix- 
tures, 149 

free-flowing,  see  Free-flowing  steam  or 
Steam,  forms  of,  above 

heat  content  in  various  types,  156-57 

movement,  see  Steam  movement  through 
soil,  below 

pressure,  see  Pressure  steam  for  soil  treat- 
ment or  Steam,  forms  of,  above 

pressureless,  from  boiler,  see  Free  flowing 
steam;  in  soil,  129,  149,  158,  191 

quantity  of  soil  heated  per  pound,  16,  132- 
33,  159-60 

saturated,  300 

specific  heat,  301 

superheated,  see  Steam,  forms  of,  above 

tunnel,  211 

volume  required  for  soil  treatment,  16,  132- 
33,  159-60 
Steam-air  mixtures  in  soil  treatment,  127,  139, 

148,  184 
Steam/air    ratio,    and    condensation,    146-48, 

149-51,  152;  relation  to  width  of  condensa- 
tion zone,  146-49,  150,  152 
Steam-chemical  (type  26),  148,  165,  166,  184- 

85,213 
Steam-generating  equipment,  191-95 

boiler  horsepower  rating,  159-60,  193-94, 
298,  300 

boiler  output,  298 

built-in,  174-76,  184 

cost,  135-37 

distribution  system,  129,  132,  136,  191,  194, 
195-96,  299 

flash  steamers,  192,  302 

fuel,  16,  135-36,  162,  163,  161-65,  174,  176, 
177,  184,  185,  187,  190,  191,  192,  194-95 

high  vs.  low  pressure  types,  191-92 

modified  from  hot-water  boiler,  192 

package  boilers,  302 

portable  unit,  168,  191 

possible  group-ownership,  193 

regular  boilers  vs.  flash  steamers,  159,   192 

size  boiler  required,   I,   16,   F32-37,  193-94 

sources,  302 

stationary  <">.  portable  units,  163,  192,  193, 
191 

Buperheal  vs.  free-flowing  vs.  pressure  types, 

1 58-59 

thermal  efficiency,  191,  299 
types  ol  boilers,  159,  192,  302 
watei  softeners,  192 


Steam  movement  through  soil,  149-51 

atmospheric  pressure  of,  129,  149,  158,  191 
condensation  zone,  128,  146,  149,  150,  152 
condensing  capacity  of  soil,  149-52,  154-55 
distance    and    flow    rate   are   compensating, 

130-31,  149-53,  156 
effect  of  clods,  15,   127,   133,   134,   140,   149, 

160-61,  190 
expanding  spheroid  around  outlet,  128,  131, 

151,  152,  153-55 
heat  transferred  by  expelled  air,  148,  150 
movement  through  pores,  89,  149 
relation  to  expanding  volume  of  spheroid, 

128,    153-55;    to   organic-matter    content, 

149;  to  soil  compaction,  134,  142,  149,  155; 

to   soil   moisture,    134,    149,    152;    to   soil 

pores,  134,  148-49,  152 
relative  movement  horizontally   and  later- 
ally, 128,  153 
steam-air  mixture,  146-48 
with  high  flow  rate  of  steam,  152-53;  inter- 
mediate flow  rate  of  steam,  152-53;  low 

flow  rate  of  steam,  149-52 
Steam  treatment  of  containers,  19,  40,  111,  133, 

134,211-12 
Steam  treatment  of  soil  (sections  8  and  9),  123- 
61;  also  frontispiece,  1,  11,  22,  48,  111,  123, 
252,  254,  264,  280 
advantages  over  chemicals,   1,   16,   18,   123- 

25,  210;  over  dry  heat,  125-26,  146 
application  to  soil  surface,   131-32,  170-76, 

178-81,  184,  187,  188 
balanced   steaming,    130,    150,    155,    159-60; 

methods  of  achieving,  159-60 
batch  methods,  133,  162,  166,  168,  191 
benefits,  126 
best  done  in  containers,  15,  19,  48,  126-27, 

164-65,  278 
best  treatment  method,  15,  125,  197 
"blow-out"  from  soil  surface,   130-31,   149, 

152-53,  155,  161 
breakdown  of  urea-formaldehyde,  14,  118 
bulk  soil,  273 
ceiling  of  21 2°F,  125,  140 
compaction  of  soil  in  relation  to,  130,  134, 

142,  148-49,  152,  155,  156 
compared  with  chemicals,  1,  16,  18,  123-25, 

210;  with  dry  source  of  heat,  125-26,  146; 

with  hot  water,  126,  1  16-18,  152,  156-57, 

18  1 
condensation  process  in  soil,   128,   1  16,  119, 

150,  152,  154-55 
condensed  water  in  steam  line.  133,  152,  178, 

196 
conditions  precluding  steaming,   125,   133- 

34,  1  18-19,  160-61 
container  size  for  chamber  steaming,  131 
continuous-batch  equipment,   16,  166,   169, 

171,  171,  176 
cooling  soil  after  treatment,  16,  134 
cosl  of,  and  iis  calculation,  is,  121,  135-37 


|  328  | 


depth  of  inputs  in  soil,  130-31 

development,  138 

effect    on    microorganisms,    113-15,    115-19, 

120,    248-50;    on    soil    structure,    126;    on 

watering  practices,  7,  126 
effectiveness,  16,  124;  determination  of,  121, 

139 
efficiency,   different   levels   of,    135-36,    152, 

159;    factors    in,    130-32,    149-53,    164-65, 

194;  measurement,  299 
equipment  for,  see  Equipment  for  soil  heat- 
ing 
escaping  steam,   from   chamhers,    130,    131; 

from   soil   surface,    128,    149-50,    153,    155, 

159-60 
final  temperature  attained,  127,  128,  140,  163 
flow  rate,  130,  132-33,  149-55 
impractical  uses,  125,  133-34,  148-49,  160-61 
in  bulk,   162,   169;  in  containers,   162,   169, 

272;  in  home  vards,  36-37,  135,  193 
in  "pinto  tag"  certification,  48,  129 
increased  growth  response,  126 
injection  into  soil,   130-31,   153-56,   167-70. 

180-81,  185-87 
lumps  of  soil,  effect,  15,  127,  133,  134,  140, 

149,  160-61,  190 
measurement  of  effectiveness,  125 
moving  soil  mass,  16,  127,  140,  148,  162,  185- 

91 
objectives,  141 
plow,  182 
quantity  of  steam  required,  132-33,  153-56, 

159,  298 
recommended  time  and  temperature,  mov- 
ing soil,  127,  140;  stationary  soil,  128,  140 
safe  to  use,  125 
salinity  from,  53,  264 
settling  of  soil  around  steam  pipes,  156 
size  of  boiler  required,  159-60,  194 
soil  leveled  for  treatment,  130,  155 
soil  moisture  after  treatment,  135,  150-51 
soil  moisture  required,  15,  130,  133,  134,  149, 

155,  160,  193 
soil  structure,  effect  on,  125;  required,   15, 

133,  160-61,  193 
spacing  of  inputs  in  soil,  130-31,  134,  153, 

155-56,  160,  180,  182;  of  steam  pipes,  130- 

31 
stacking  flats,  131,  189 
stationarv   soil   mass,    16,    128,    132-33,    140, 

162,  163,  166-77 
steam-air-chemical  mixtures,  125,  148 
steam-air  mixtures,  127,  139,  148 
steam  flow  rate,  most  efficient,  130,  149-55 
temperature,  and  time  required,  15,  16,  113- 

15,  124,  127-29,  139-40,  159,  163;  in  mov- 
ing  mass,    127;    in   stationary   mass,    127; 
measuring,  156 
time,  per  cu.  yd.,  159,  194 
toxicity,  see  Toxicitv  after  treatment  of  soil 
"trickle  finish,"  128,130,  132,  155,  173 


types  of  fuel,  see  Fuel  for  steam  boilers 
uneven  heating,  factors  in,  133-34,  160-61 
used  in  closed  areas,  125,  210 
used  near  living  plants,  124,  125,  135,  210 

.     watering  practices  in  treated  soil,  125 
weed  control,  15,  126,  139-40 

Steaming  soil,  equipment  for,  see  Equipment 
for  soil  heating 

Steel,  specific  heat,  301 

Steer  manure,  see  Manure 

Stellaria  (chickweed),  235 

Stem  and  bulb  nematode  [Ditylenchus),  47, 
227;  lethal  temperatures,  139;  variability 
in  pathogenicitv,  258 

Stem  and  leaf  gall,  oleander,  220 

Stem  rot,  4,  35,  36;  see  also  Damping-off 

Stemphylium,  233 

Stericover  for  soil  treatments,  178,  303 

Sterilite,  for  soil  treatments,  303 

Steriltex  cover  for  soil  treatments,  178,  303 

Sting  nematode  (Belonolaimus),  261 

Stock  (Matthiola),  111,  215,  219,  263;  am- 
monium injury,  112,  113;  bacterial  blight, 
218,  229,  261;  Botrytis  blight,  8,  9,  55;  hot- 
water  treatment  of  seed,  229;  mosaic,  235; 
Rhizoctonia  foot  rot,  42,  43,  207;  salinitv 
injury,  7,  8,  9,  55,  58;  Sclerotinia  white 
blight,  cottony  rot,  46;  Verticillium  wilt,  256 

Stoddard  solvent,  sources,  305;  toxicity  to 
plants,  215;  used  with  copper  naphthenate, 
19,213 

Stone-fruit  viruses,  232 

Storage  of  soil  mixes,  13,  71-75,  275 

Strains  of  a  pathogen,  determination,  6,  256- 
59;  differences  between,  6,  43;  effects  on 
disease,  6,  256-59;  relation  to  pathogen-free 
stock,  43 

Strawberry,  220 

Strelitzia,  229 

S:reptom\ces  as  a  retardant,  25,  251;  effect  of 
pHon,251 

Streptomycin,  255-56 

Stunting  of  plant,  from  salinity,  8,  42,  55; 
from  soil  toxicitv,  9,  79,  95,  111 

Subirrigation,  relation  to  salinitv,  63,  119,  183 

Substrate,  301 

Succulents,  20,  29,  37 

Sugar,  241 

Sid  fate  in  water,  64 

Sulfate  of  potash,  see  Potassium  sulfate 

Sulfur,  bacteria,  246-47;  evele,  246-47;  essen- 
tial to  plants,  89,  106;  rendered  available  by 
microorganisms,  237,  246-47;  soil  acidifi- 
cation, 246-47 

Sulfuric  acid,  247 

Summary,  general  (Section  1),  3-27 

Superheated  steam,  16,  135.  211.  301;  advan- 
tages, 130,  158-59,  166;  equipment  for  gen- 
erating, 192;  heat  content,  130.  157-59,  166; 
thermal  efficiency.  130,  157-59;  see  also 
Steam,  forms  of  soil  treatment 


[  329  ] 


Supernatural  cause  of  disease,  3,  34 
Superphosphate,  single  and  double,  14,  70-75, 

76,  77,  91,  94,  106;\reble,  see  double,  above; 

use  finely  ground  product,  70 
Support,  as  a  soil  function,  10,  87 
Suppression  of  damping-off  is  not  control,  6, 

23,  36-37,  43-44,  45 
Surface  application  of  organic   fertilizer,  ad- 
vantages, 13-14,  119,  121,  122;  disadvantages, 

122 
Surface  methods  of  soil  steaming,  131-32,  170— 

76,  178-81,  184,  187,  188 
Survival   of  microorganisms   in   soil,  21,  238, 

243,  261 
Susceptibility  of  host,  to  ammonium  injury, 

13,  96,  111,  112,  115;  to  damping-off,  5,  42; 

to  disease,  5,  300;  to  nematodes,  258-59;  to 

root  rot,  256-58;  to  salinity,  7,  55,  61 
Sweet  alyssum  (Lobularia),  13,  111,  112,  113, 

264 
Sweet  basil,  205 
Sweet  pea,  seed  decay,  230 
Sweet  potato,  221 
Syngonium,    black    cane    rot,    229;    hot-water 

treatment  of  plants,  227,  229-30;  effect  on 

dormancy,  229-30 
Systox  (Demeton),  232,  304 

Tagetes,  starter-solution  tests,  111;  Rhizoc- 
tonia  on,  256 

Tanks,  hot- water- treating,  302 

Tax  rates  increasing,  effect  on  nurseries,  31, 
271,274 

Tear  gas,  see  Chloropicrin 

Temperature,  29;  lethal  to  microorganisms 
and  weeds,  127,  139-40;  recommended  for 
soil  steaming,  15,  16,  113-15,  124,  127-29, 
139-40,  159,  163;  relation  to  plant  distribu- 
tion, 86;  relation  to  variability  of  micro- 
organisms, 259;  see  also  Tempil  pellets  and 
Thermometers 

Tempil  pellets  for  temperature  measurement, 
129,  156,  303 

Terraclor  (PCNB),  19,  43,  205,  207,  208-9;  ef- 
fectiveness, 205;  sources,  305;  specificity,  205 
I  ersan  (thiram),  19,  43,  207,  209,  305 

Thermometers,  128-29,  156,  302;  expanded- 
scale  type,  225,  302;  precision  type  for  check- 
ing those  used,  224-25,  302;  see  also  Tempil 
pellets 

Thielaviopsis  basicola,  variability  of  patho- 
genicity, 258 

Thiram    (Arasan,   Tersan),    19,   43,   207,   209; 

sources,  305 
I  hornas  method  lor  soil  steaming,  163;  effec- 
tive depth,  131,  178;  equipment,  178-79; 
for  beds  or  benches  (surface)  (type  18),  Hi, 
131,  132,  11H,  163,  165,  166,  171,  178-80; 
movable  variant,  118,  Kit;,  180;  lor  contain- 
ers (type  ")),  Hi,  131,  132,  118,  164,  166,  170- 
71,  179,272 
I  hrips,  'J 


Tip  cuttings  to  eliminate  pathogens,  20,  219- 

21 
Tipburn,  salinity,  8-9,  55,  58,  60 
Tobacco,  82,  214,  215,  248;  mosaic,  140;  Phy- 
tophthora  root  rot,  258,  261;  Thielaviopsis 
on, 258 
Tomato,  25-26,  29,  111,  260,  264-65;  broom- 
rape  on,  261;  crown  gall,  258;  Fusarium  wilt, 
257;  Rhizoctonia  on,  41,  42,  256;  root-knot 
nematode  on,  259;  spotted  wilt,  235;  Verti- 
cillium  wilt,  256 
Tools,  relation  to  spread  of  pathogens,  6,  40- 
41,  45,  48,  217,  249;   treatment,  19,  22,  23, 
24,40,48,  201,226 
Top  rot  of  seedlings,  4,  35,  37,  38 
Toxicity  after  treatment  of  soil,  frontispiece, 
1,9-11,90,93,98,  124,  140 
caused  by  ammonium,  9,  13-14,  79,  95,  96, 
98,  111-13,  115,  121,  122,  199,  253-54;  by 
manganese,  9,  95,  98;  by  other  agents,  95; 
by  salinity,  9,  95,  98;  by  soluble  organic 
matter,  9-10,  95,  98 
effects  on  plants,  9,  94-95 
factors  affecting  type  and  severity,  94 
persistence,  95 

reduction  by  aging  soil,  10,  93,  95,  96;  by 
immediate  planting,  10,  95,  96;  by  leach- 
ing soil,  10,  95,  96;  by  steaming  ingredi- 
ents separately,  93;  by  using  U.  C.-type 
mix,  9-10,  11,   12,   15,  90,  93,  96,  97-98, 
124,  129,  140,  199,  270 
residue  of  chemicals,  see  Chemical  residue 
Transit  mixers,  see  Concrete  mixers 
Transpiration,  54—55,  56-57,  87 
Transplanting,  25,  81,  264,  282-83 
Transplants,  soil  mixes  for,  71,  72,  74 
Treated   soil,   see   Chemical   residue;    Recon- 
tamination    problem;    and    Toxicity    after 
treatment  of  soil 
Treatment,  see   Chemical    treatment  of   soil; 
Containers,    treatment    of;    Dry    source    of 
heat  for  soil  treatment;  Hot-water  treatment 
of   soil;    Obtaining   pathogen-free   planting 
stock;   Steam  treatment  of  soil;  and  Tools, 
treatment 
Trees,  29 
Trichoderma  as  a  retardant,  25,  240,  251-52; 

effect  of  pH  on,  251 
"Trickle  finish"  in  soil  steaming,  128,  130,  132, 

155,  173 
Trifolium  (clover),  259 
Triton  B-1956,  spreader,  232,  305 
Tropaeolum  (nasturtium),  215,  224,  235 
"Trough"  soil  steamer,  176 
Tub  containers,  131 

Tubular  soil  heater  with  electric  elements,  177 
l  ufedge  (over  for  soil  treatments,  178,  303 
lung,  Rhizoctonia  on,  257 

linl,  for  soil  mixes,  91,  93,  110;  source  of 
composl  variability,  10.  89;  unavailability, 
12,90,  93 


|  330  | 


I'.C.L.A.   blend,  93;   see  also   U.  C.-type  s:»ii 

mixes 
U.  C.  system   (Section    1),  3-27;  also   frontis- 
piece,'27,   123;  advantages,  1,  3,  30-33,  49, 

51,  270;  aids  in  adopting,  1-2;  explanation, 

3,  4;   grower  experience,  263-70;  mechani- 
zation, 271 
U.  C.-type  soil  mixes  (sections  5,  6),  68-107; 

also  frontispiece,   1,   10-13,  93-94,   123,   197 

adoption,  1-2,  27,  79,  263-64 

advantages,  1,  10-12,  89-90,  93-94,  96,  265, 
270 

aeration,  87,  270 

application  to  bench  and  bed  crops,  12,  69, 
80-81,  268;  to  can  plants,  69,  72,  80,  267; 
to  cvmbidiums,  268-69;  to  flatted  plants, 
12,  69,  72,  75,  79,  264-65;  to  foliage  plants, 
20,  29,  32,  266-67;  to  home-yard  planting, 
81;  to  planter  boxes  and  dish  gardens,  81, 
84;  to  pot  plants,  12,  69,  73,  77,  79-80, 
266-67;  to  research,  81-83;  to  vegetable 
plants,  265-66 

base  exchange,  99 

centralized  soil  service,  193 

components,  see  Ingredients  for  U.  C.-type 
mixes 

composting  eliminated,  89-90,  93,  270 

conductance,  65,  70,  266 

cost,  12,  69,  80,  84-85,  100 

cultural  practices  modified  by  use,  27 

development,  93-94,  110 

dump  soil,  re-use,  84 

enable  scheduled  production,  94 

evaluation,  263-70 

facilitate  mechanization,  25,  90,  94,  270,  271 

fertilizers  included  in  mixes,  69-76;  see  also 
Fertilizer  ingredients 

fertilizing,  see  Fertilizers,  dry,  and  Fertili- 
zers, liquid 

formulas,  76-79 

leachabilitv,  65-67 

micronutrients,  12,  89,  101,  106-7,  109,  110 

mixes  A  to  E,  69-76;  see  also  Soil  mixes 

mixing,  83-84,  97,  98,  105,  133,  276-79 

moisture  retention,  12,  69,  87,  97,  100,  270 

nontoxicity  after  treatment,  9-10,  11,  12,  15, 
90,  93,  96,  97-98,  124,  129,  140,  199,  270 

permeability,  64-65 

pH,  69,  70 

plant  growth,  27,  265-66,  270 

preparation,  83-84,  97,  98,  105,  133,  276-79 

reduce  labor,  10,  84,  89-90,  100,  270;  space 
requirement,  10,  12,  89-90,  100;  odor  and 
fly  problems,  12,  90,271 

reliability,  68,  270 

reproducible,  10,  89-90,  93 

retention  of  nutrients,  12,  97,  99 

root  distribution  in,  82-83 

salinity  problem  reduced,  10,  15,  53,  64,  65- 
67,90,211,265-66,270 

shrinkage  in  storage,  12,  80 


transplanting  to  clay  soil,  81 
types  planted  within  a  week,  71-75,  275,  278 
types  stored  before  use,  13,  71-75,  275 
uniform  initial  fertility,  12,  98-100 
uniform  materials,  12,  97-98 
uniform  results,  12,  68,  89-90,  270 
uses,  12,  69,  79-83,  264-70 
variants,  69-76,  93 

water  content,  12,  69,  87,  97,  100,  270 
watering,  27,  81;  adjustment  when  first  us- 
ing, 79,  84 
weight,  per  cu.  ft.,  12,  69,  80 
Ultron  cover  for  soil  treatment,  302 
Uneven  soil  heating  with  steam,  causes,  134 
Unit  containers  for  marketing,  25,  31,  127 
Unsteady  state  in  heat  transmisison,  145-46 
Urea,  13,  14,  78,  79,  105,  106,  108,  109,  118,  245 
Urea-formaldehyde  resins,   13,   14,  79;   biuret 
content,  13,  14,  78,  79,  119;  effect  of  steam- 
ing, 14,  118;  use  as  fertilizers,  105,  115-19, 
121 

Vacuum-plate  seed  planters,  25-26,  264,  283, 
302 

Vapam,  18,  204-5,  208;  application,  18,  204-5; 
effectiveness,  204;  sources,  305 

Variability  of  pathogens,  255-59 

Vault  for  soil  steaming  (type  6),  16,  25,  131, 
132,  148,  164,  166,  167,  171-72,  174,  272,  277, 
280-81 

Y-C  13,  206,  305 

Vegetable  plant  production,  29,  129;  U.  C. 
system  for,  111,  264-66 

Velon  Fumicover  for  soil  treatments,  178,  180, 
303 

Venturi  tube,  in  steam-air  mixing,  148 

Verbena,  86,  11 1,  1 13,  264 

Vermiculite,  12,  94,  97,  100,  101 

Vertical  cabinet  for  soil  steaming  (type  8),  131. 
132,  148,  164,  166,  173-75,  176;  self-generat- 
ing type,  electric  (type  11),  131,  164,  174, 
175-76,  195 

Verticillium  albo-atrum,  198,  220,  222,  239-40, 
250;  survival  in  soil,  261;  variability  of  para- 
sitism, 256 

Verticillium  wilts,  7,  18,  31,  49,  201,  203,  219, 
235;  cultured-cutting  technique  against. 
221-22;  ineffectiveness  of  methyl  bromide 
against,  16,  18,  22,  124,  203 

Vigna  (cowpea),  95 

Yinca,  111 

Viola,  see  Pansy  and  Violet 

Violet,  263 

Virulence  of  pathogens,  5,  6,  260,  300;  varia- 
bility in.  256-59 

Viruses,  299;  elimination  by  culturing  grow- 
ing point,  221;  elimination  by  use  of  true 
seed,  7,  21,  232-34;  inactivation  by  soil  heat- 
ing, 140;  indexing,  232-33;  survival  in 
refuse,  140;  survival  in  soil,  140 

Viscosity,  of  gases,  143;  of  steam,  143 


[331] 


Visqueen  cover  for  soil   treatments,   178,  201, 

302,  303 
Volatilization  of  chemicals  for  soil  treatment, 

148,  165,  166,  184-85,213 
Volume  of  steam,  16,  132-33,  149-52,  159-60 

Water 

application  by  porous  hose  or  drip  system, 
81 

calcium  and  magnesium  content,  64 

changes  form  at  different  temperatures,  156 

conductance,  9,  14,  63 

deficit,  cumulative  effect  on  plant,  87-88 

deionized,  15,  63-64 

heat  conductivity,  299 

leaching  with,  see  Leaching  to  reduce  sa- 
linity 

loss  by  transpiration,  54-55,  56-57,  87 

necessary  to  use  excess,  9,  14,  63 

organisms  spread  in  water  supply,  5,  11,  38, 
45,48 

quality,  in  relation  to  salinity,  9,  11,  14,  30, 
63-64;  characteristics,  63 

retention  by  U.  C.-type  soil  mixes,   12,  69, 
87,  97,  100,  270 

salt  deposit  on  leaves  from,  9,  63-64 

sodium  content,  64,  106,  109 

solvent  for  minerals,  87 

source,  of  micronutrients,   107;   of  salinity, 
9,  11,  30,53-54,  192 

specific  heat,  301 

supplied  by  soil,  10 

use  in  plant  metabolism,  87 
Water  breakers  on  hoses,  84 
Water-culture  growing  of  plants,  87 
Water-mold    root    rots,    caused    by    Pythium 

and  Phytophthora,  7,  35-36,  43,  44-46,  49- 

50,  82,  135,  139,  220,  226-27,  229,  233,  235, 

250,  261;  favored  by  very  wet  soil,  36,  43, 

44,  49-50;  symptoms,  35-36,  44-45;  see  also 

Damping-off 
Water   molds,  5,   22,   38,  49-50,   220-21,  238, 

301;   lethal   temperatures,   139;  life  history, 

44-45;    retard    root    development,    45;    sur- 
vival in  soil,  261 
Water  requirements  of  plants,  51,  87 
Water  softeners,   for   boilers,    192;    unsuitable 

lor  plant  use,  64 
Water  spotting,  9,  63-64 


Water  trap  in  steam  lines,  133,  196 

Watering  of  plants,  in  steamed  soil,  126;  in 
U.  C.-type  mixes,  27,  79,  81,  84;  mechan- 
ized, 27,  283-84;  relation  to  disease,  43,  49 

Watermelon,  root-knot  nematode  on,  259 

Weed  hosts  of  viruses,  235,  275 

Weed  killers,   18 

Weeding  in  containers,  cost  of,  126,  264; 
elimination  by  soil  treatment,  126,  264-65, 
269-70 

Weeds,  261,  264,  275;  control  by  soil  treat- 
ment, 15,  18,  124,  126,  138,  197,  198,  200, 
202-3,  204,  208,  248;  lethal  temperatures 
of  seed,  127,  139-40 

Weigelia,  hot-water  treatment  of  plants,  229; 
root-knot  nematode  on,  229 

Weights,  table  of,  301 

Wheat  straw,  100,  254 

Wheatstone  bridges,  see  Solubridge  for  sa- 
linity measurement 

White  blight,  see  Sclerotinia  cottony  rot 

Wild-oat  seed,  heat  resistant,  139 

Wild  radish,  stock  mosaic,  235 

Wilting,  plant,  87;  root  rot,  36;  salinity  in- 
jury, 9,  55 

Wire-stem  damping-off,  5,  35-36,  43 

Wolman  salts  for  wood  preservation,  215,  216 

Wood,  specific  heat,  301 

W^ood  shavings,  see  Shavings,  wood 

Woody  plants,  can-grown,  U.  C.  system  for, 
69,  72,  80,  267 

Worms,   lethal    temperature,   139 

Xiphinema   (dagger   nematode),  229 

Year-round  growing,  3,  7,  30-31,  49;  under 
glass,  7 

Zantedeschia,  see  Calla 

Zea  (corn),  240-41 

Zinc,  95,  237;  essential  to  plants,  89,  106;  role 

of  microorganisms  in  deficiency  of,  248 
Zineb  (Parzate),  267;  sources,  305 
Zinnia,  111;  Alternaria  disease,  218,  229,  233; 

hot-water  treatment  of  seed,  229;   Rhizoc- 

tonia  in  seed,  42,  229 
Zoning  restrictions  against  nurseries,  31 
Zoosporangia,  44-45 
Zoospores,  44-45 


Cooperative  Extension  work  in  Agriculture  anil  Home  Economics  College  .,(  v  n.  uJiure,  University  of  California,  and  United  Statei  Department  of  Agriculture 
co  operating.  Diatribuled  In  IuiiIhi.hu.-  of  tin-  ,\<  i  <>i  <  <m;  >.     ..(  May  h,  un.l  Jon.-  30,  1914  George  1!.  Alcorn,  Director,  California  Agrii  ultural  Extei Service. 


lm  •<::>!<  B8163  ;.\lk 


E   ON   THE   LAST   DATE 
"D    BELOW 

No  matte         lat  kill     ,  Ac         THER  BORRQ,      tave  .  .  . 

...  if  you  employ  usual  nursery  pr  CT1ER  ONE  Wfs,  this  manual  shows  you  how  to 
cut  your  costs  and  losses,  and  incre  'O 

The  nursery  program  it  offers 

can  help  you  to  cut— maybe  even  eliminate— losses  from  diseases  and  weeds,  and  at  the 
same  time  cut  the  cost  of  fighting  them!  The  program  is  the  result  of  16  years'  research 
by  the  Department  of  Plant  Pathology,  University  of  California,  Los  Angeles.  The 
methods  have  been  thoroughly  tested  in  commercial  nurseries,  not  just  in  the  laboratory. 

The  U.  C.-type  soil  mixes 

are  basic  to  the  program.  Besides,  they  offer  you  worth-while  savings  in  labor  costs 
and  storage  space,  not  to  mention  surer  and  probably  faster  results. 

Mechanizing  your  operations 

can  further  cut  labor  costs.  With  an  effective  disease-prevention  program  and  a  U.  C.- 
type  soil  mix,  mechanizing  has  become  a  fact.  The  manual  describes  mechanization  as 
it  has  developed  in  a  number  of  California  nurseries. 

IK 

Your  nursery  is  unique.  **" 

Your  problems  are  somewhat  different  from  anyone  else's.  |VA  nursery  business  in 
California  is  a  varied  one,  and  no  rules  of  thumb  will  applf'to  all  the  hundreds  of 
crops  grown  here.  Neither  will  detailed  directions  cover  all  future  developments  in  the 
industry.  Therefore  the  manual  presents  a  general  program,  explains  the  facts  behind 
it.  It  gives  many  down-to-earth  examples.  It  describes  and  illustrates  the  various  kinds 
of  equipment  you  might  use.  With  this  background  and  the  knowledge  of  your  prob- 
lems, the  program  can  be  adapted  to  your  needs,  now  and  in  the  future. 

For  your  convenience,  nn 

references  where  you  can  find  still  further  information  on  specific  points,  a  glossary, 
some  methods  of  computation,  a  table  of  weights  and  measures,  and  sources  where 
you  can  get  equipment  and  materials  are  given  in  an  appendix.  An  index  is  provided 
for  rapidly  locating  information. 


,,*'■  '  - 


To  obtain  additional  copies  of  this  manual  or  a  catalog  listing  other  manuals  arid  free  publica- 
tions, see  your  University  of  California  Farm  Advisor  (offices  located  in  most  California  counties), 
or  write  to: 

Agricultural  Publications 
22  Giannini  Hall 
University  of  California 
Berkeley  4,  California 

When  ordering  manuals,  send  orders  and  payment  to  the  address  above;  make  checks  or  money 
orders  payable  to  The  Regents  of  the  University  of  California.